Method for removing nitrate from high-salinity wastewater by electrochemical reduction

Abstract

The present invention belongs to the technical field of wastewater treatment, and specifically relates to a method for removing nitrate from high-salinity wastewater by electrochemical reduction. The present invention prepares a titanium-based oxide coating electrode by doping with rare earth elements, and uses phosphorus and / or sulfur-containing inorganic salts or organic matter as additives to improve the selectivity of electrochemically reducing nitrate to generate nitrogen and / or ammonia, thereby having high selectivity in removing nitrate from high-salinity wastewater, and can electrochemically reduce nitrate to nitrogen and / or ammonia for removal, and the ammonia can be further recovered as nitrogen fertilizer.

Description

Technical Field

[0001] The invention relates to the technical field of wastewater treatment, and in particular to a method for removing nitrate from high-salinity wastewater by electrochemical reduction using a rare earth element-doped cathode. Background Art

[0002] High-salt wastewater is mainly composed of sodium chloride, sodium sulfate, sodium nitrate and other salts. The by-product salt produced by evaporation and crystallization contains not only other salt impurities, but also a small amount of refractory organic matter, calcium and magnesium hardness, other heavy metal ions and Si and other impurities. Not only is it of low value and cannot be directly used, but large-scale storage is also prone to secondary pollution, so it needs further resource treatment.

[0003] Membrane salt separation is generally used to separate monovalent salts and divalent salts in high-salt wastewater. It can only be used to remove sodium sulfate but cannot effectively separate monovalent salts sodium chloride and sodium nitrate. Since sodium nitrate has a high solubility and almost all nitrates are easily soluble in water, it is difficult to achieve efficient separation and deep removal of nitrates in high-concentration NaCl wastewater using traditional freezing crystallization technology.

[0004] Currently, the methods for removing nitrate from wastewater include ion exchange, biological denitrification, chemical denitrification and physicochemical methods, and many researchers have carried out special research and reports.

[0005] Some researchers have proposed a method for treating nitrate wastewater. For example, patent application CN202210036483.5 discloses the use of an electroactive nitrate-reducing bacteria-zero-valent iron coupling system to treat nitrate wastewater. This method can avoid the dependence of nitrate-reducing bacteria on organic carbon sources and achieve efficient and directional conversion of nitrate in wastewater to produce ammonia. However, the high concentration of NaCl salt in coal chemical brine will limit the activity of nitrate-reducing bacteria.

[0006] Other removal of NO3 from high-salinity wastewater - Ion methods, such as ion exchange method, exist NO3 - With Cl - Competitive adsorption between ions, due to the lack of NO3 - Ion selective adsorption resin makes it difficult to adsorb and remove nitrates in NaCl high-salt wastewater; the traditional chemical denitrification method removes nitrates through active metal denitrification and catalytic denitrification, but it will bring in other impurities; the traditional physical and chemical method refers to the use of reverse osmosis + electrodialysis technology, but it cannot achieve NO3 - With Cl - Separation of ions.

[0007] Therefore, it is necessary to further study new methods that can achieve deep removal of nitrates from high-salt wastewater in coal chemical industry. Summary of the invention

[0008] The object of the present invention is to provide a method for electrochemical reduction to remove nitrates in high-salt wastewater, aiming to provide a cathode doped with a rare earth element having high catalytic activity and high selectivity for nitrates in high-salt wastewater, thereby efficiently and directionally removing nitrates in high-salt wastewater through an electrochemical reduction reaction.

[0009] In order to achieve the above-mentioned object of the invention, the present invention provides, on the one hand, a method for removing nitrate from high-salinity wastewater by electrochemical reduction, which comprises the following steps:

[0010] providing an anode and a rare earth element doped cathode;

[0011] Adding high-salt wastewater into the electrolytic cell, and performing an electrochemical reduction reaction with the anode and the rare earth element-doped cathode to reduce the nitrate into nitrogen and / or ammonia for removal;

[0012] Wherein, the preparation method of the rare earth element doped cathode comprises:

[0013] Providing a titanium-based metal electrode and a precursor solution, wherein the precursor solution includes a metal oxide, a salt containing a rare earth element, and an inorganic salt or organic matter containing sulfur and / or phosphorus;

[0014] Applying the precursor solution on the surface of the titanium-based metal electrode and performing a sintering treatment;

[0015] After constant temperature crystallization treatment, a rare earth element doped cathode is obtained.

[0016] In another aspect, the present invention provides a method for preparing a rare earth element-doped cathode, comprising:

[0017] Providing a titanium-based metal electrode and a precursor solution, wherein the precursor solution includes a metal oxide, a salt containing a rare earth element, and an inorganic salt or organic matter containing sulfur and / or phosphorus;

[0018] Applying the precursor solution on the surface of the titanium-based metal electrode and performing a sintering treatment;

[0019] After constant temperature crystallization treatment, a rare earth element doped cathode is obtained.

[0020] In another aspect of the present invention, a rare earth element doped cathode is provided, which is prepared by the method for preparing a rare earth element doped cathode of the present invention.

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

[0022] 1) The method for removing nitrate from high-salinity wastewater by electrochemical reduction provided by the present invention comprises providing a new rare earth element-doped cathode, which is based on the unfilled 4f shell of the atoms or ions of the rare earth element so that it has a high electron transfer activity, preparing an electrode with a titanium-based oxide coating by rare earth element doping, and using phosphorus and / or sulfur-containing inorganic salts or organic matter as additives to improve the selectivity of electrochemical reduction of nitrate to generate nitrogen and / or ammonia, thereby having high selectivity in removing nitrate from high-salinity wastewater, and can electrochemically reduce nitrate to nitrogen and / or ammonia for removal, and the ammonia can be further recovered as nitrogen fertilizer.

[0023] 2) The method for removing nitrate from high-salt wastewater by electrochemical reduction provided by the present invention can deeply remove nitrate from high-salt wastewater, has the advantages of high efficiency and low energy consumption, can be widely used for the efficient removal of nitrate from high-salt wastewater generated in various industrial fields, and has broad and good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of the method for removing nitrate from high-salt wastewater by electrochemical reduction provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0025] In the present invention, "high-salt wastewater" is the concentrated brine produced by membrane salt separation and concentration of coal chemical wastewater, the centrifuged mother liquor of NaCl industrial salt removed by evaporation, concentration and crystallization, and the product after organic matter, calcium and magnesium ions, heavy metals and Si impurities are removed to reduce electrode side reactions. In some embodiments, the salt content TDS of the high-salt wastewater is >100g / L, mainly containing NaCl and a small amount of nitrate, wherein the nitrate ion concentration is about 1000mg / L.

[0026] According to the principle of electrochemical reduction of nitrate, NO3 - Ions can be reduced to NO2 - , N2 and NH4 + As well as products such as NO, N2O, NH2OH, etc. The main cathode reactions involved in the electrochemical reduction of nitrates are as follows:

[0027] NO3 - +H2O+2e - →NO2 - +2OH - (1)

[0028] NO2 - +2H2O+3e - →1 / 2N2+4OH - (2)

[0029] NO2 -+5H2O+6e - →NH3+7OH - (3)

[0030] NO2 - +4H2O+4e - →NH2OH+5OH - (4)

[0031] 2NO2 - +3H2O+6e - →N2+8OH - (5)

[0032] 2NO2 - +3H2O+4e - →N2O+6OH - (6)

[0033] NO2 - +H2O+2e - →NO+2OH - (7)

[0034] N2O+5H2O+4e - →2NH2OH+4OH - (8)

[0035] 2H2O+2e - →H2+2OH - (9)

[0036] The electrochemical reduction of nitrate involves the steps of diffusion, adsorption, charge transfer, desorption and separation of reactants, among which the adsorption of nitrate on the active site is the first step of electrochemical reduction, and the electrode material with catalytic activity is the key to realize the electrochemical reduction of nitrate.

[0037] The inventors of the present invention have found in their research that copper electrodes have higher activity for nitrate reduction in neutral and alkaline media, but their products are mainly nitrite, hydroxylamine and ammonia. At the same time, compared with pure metals, the synergistic effect between components such as platinum-based bimetallics such as Pt-Ir, Pt-Sn and Pt-Pd makes it have higher nitrate reduction activity, and the main product in acidic media is ammonia. By modifying or alloying platinum metals with Cu, the final product can be converted into N2. However, since this type of electrode requires the use of precious metals, the electrode cost is high, making it impossible to use it widely. In addition, when the existing electrodes are used for electrochemical reduction of nitrates, there are still problems such as low efficiency of electro-reduction of nitrate removal, high energy consumption, more electrode side reactions, and generation of a variety of nitrate reduction intermediates, as well as cathode hydrogen evolution reactions in the electrochemical system, which compete with the desired electro-reduction of nitrate removal process, so that the electro-reduction of nitrogen removal process cannot proceed smoothly. The core reason for these problems is the lack of selective high catalytic activity electrodes, which leads to the inability to carry out the electro-reduction of nitrate removal in high-salinity wastewater in a directional and efficient manner.

[0038] In order to solve the above problems, the present invention is based on the fact that the unfilled 4f shell of the atoms or ions of rare earth elements has a higher electron transfer activity, and prepares a titanium-based oxide coating electrode by doping with rare earth elements, and uses phosphorus and / or sulfur-containing inorganic salts or organic matter as additives to improve the selectivity of electrochemical reduction of nitrates to generate nitrogen and / or ammonia, so that it has high selectivity in the removal of nitrates from high-salinity wastewater, and can electro-reduce nitrates to nitrogen and ammonia for removal, and ammonia can be further recovered as nitrogen fertilizer, so as to achieve efficient removal of nitrates in high-salinity wastewater. Furthermore, the present invention also provides a precursor solution with an optimized ratio, which, combined with repeated coating-high-temperature sintering treatment and specific sintering conditions and crystallization treatment conditions, can further improve the selectivity and catalytic activity of electrochemical reduction of nitrates, and thus completely remove nitrates from high-salinity wastewater.

[0039] Specifically, an embodiment of the present invention provides a method for removing nitrates from high-salinity wastewater by electrochemical reduction, which comprises the following steps:

[0040] S1, providing an anode and a rare earth element doped cathode;

[0041] S2, adding high-salt wastewater into the electrolytic cell, and performing an electrochemical reduction reaction with the anode and the rare earth element-doped cathode to reduce the nitrate into nitrogen and / or ammonia for removal;

[0042] Wherein, the preparation method of the rare earth element doped cathode comprises:

[0043] S11, providing a titanium-based metal electrode and a precursor solution, wherein the precursor solution includes metal oxides, salts containing rare earth elements, and inorganic salts or organic substances containing sulfur and / or phosphorus;

[0044] S12, coating the precursor solution on the surface of the titanium-based metal electrode and performing a sintering treatment;

[0045] S13, and then subjecting the cathode to be doped with a rare earth element through a constant temperature crystallization treatment.

[0046] Specifically, in S1, the rare earth element doped cathode is a titanium-based metal oxide coating electrode doped with rare earth elements and containing sulfur- and / or phosphorus-containing inorganic salts or organic matter as additives, and its preparation method is shown in S11 to S13.

[0047] Wherein, in S11, the titanium-based metal electrode is a titanium mesh or titanium plate of appropriate size. In some embodiments, the size of the titanium mesh or titanium plate is 10*15 cm 2 wait.

[0048] The precursor solution includes metal oxides, salts containing rare earth elements, and inorganic salts containing sulfur and / or phosphorus. In some embodiments, the metal oxide is selected from at least one of Sb-SnO2, RuO2, and IrO2.

[0049] In some embodiments, the salt containing rare earth elements is a chloride, sulfate and / or nitrate of La, Ce and / or Pr. In some specific embodiments, the salt containing rare earth elements is lanthanum chloride or cerium chloride.

[0050] In some embodiments, the sulfur- and / or phosphorus-containing inorganic salt or organic matter is used as an additive. In some specific embodiments, the sulfur- and / or phosphorus-containing inorganic salt or organic matter is sodium sulfate or sodium phosphate.

[0051] In some preferred embodiments, the molar concentration ratio of the metal oxide, the salt containing rare earth elements, and the sulfur- and / or phosphorus-containing inorganic salt or organic matter is 100:(0.05-1):(0.01-0.1).

[0052] In some embodiments, the titanium-based metal electrode is also subjected to surface pretreatment. Preferably, the surface pretreatment includes at least one of degreasing, removing the oxide layer, and forming a rough surface. In some preferred embodiments, the degreasing is to use alkaline solution, organic solvent and deionized water to wash the titanium-based metal electrode in sequence, and repeat at least three times; the oxide layer is removed by graded grinding with coarse sandpaper and fine sandpaper until the oxide layer on the surface of the titanium-based metal electrode is completely removed; the rough surface is formed by soaking the titanium-based metal electrode in 5%-10% oxalic acid solution for 12-48 hours; for example, soaking for 12 hours, 15 hours, 18 hours, 20 hours, 24 hours, 30 hours, 36 hours, 40 hours, 42 hours, 48 ​​hours.

[0053] In S12, the precursor solution is coated on the surface of the titanium-based metal electrode. Specifically, various components in the precursor solution (i.e., a solution containing the metal oxide, a solution containing the salt containing the rare earth element, and a solution containing the sulfur- and / or phosphorus-containing inorganic salt or organic matter are sequentially coated) are sequentially layered and coated on the surface of the titanium-based metal electrode (preferably, the titanium-based metal electrode is surface pretreated), or various components in the precursor solution are mixed and then coated on the surface of the titanium-based metal electrode (preferably, the titanium-based metal electrode is surface pretreated).

[0054] In some embodiments, the sintering process is carried out in an oxygen-rich atmosphere at a pressure of 0.2-2.0 MPa, with a sintering temperature of 500-1200°C and a sintering time of 30-120 min. Specifically, the O2 concentration of the oxygen-rich atmosphere is >300 mg / L; the sintering temperature can be 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C or 1200°C; the sintering time can be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min.

[0055] In some preferred embodiments, the steps of coating the precursor solution on the surface of the titanium-based metal electrode and performing sintering treatment are repeated multiple times; preferably, repeated 10-20 times.

[0056] In S13, in some embodiments, the temperature of the constant temperature crystallization treatment is 500-800° C., and the time is 12-48 hours. Specifically, the temperature of the constant temperature crystallization treatment can be 500° C., 600° C., 700° C. or 800° C.; the time of the constant temperature crystallization treatment is 12 hours, 15 hours, 20 hours, 24 hours, 30 hours, 36 hours, 40 hours or 48 hours.

[0057] In S1, the anode is an anode known in the art for electrochemical reactions, including but not limited to titanium-based Sb-SnO2 oxide coating electrodes, platinum sheet electrodes, carbon felt electrodes, and graphite electrodes, etc. In some embodiments, the anode has an area equal to that of the rare earth element-doped cathode.

[0058] In some specific embodiments, when the anode is a titanium-based Sb-SnO2 oxide coating electrode, it can be purchased directly or prepared in a similar manner to S11 to S13. During preparation, the difference from S11 to S13 is that the precursor solution does not include salts containing rare earth elements and inorganic salts containing sulfur and / or phosphorus, that is, the anode is not doped with rare earth elements and no additives (inorganic salts or organic matter containing sulfur and / or phosphorus) are added.

[0059] In S2, in some embodiments, during the electrochemical reduction reaction, the current density applied is 10-50 mA / cm 2 The pH of the high-salinity wastewater is controlled at 3-7, and the reaction time is 0.5-3h. Specifically, the current density is 10mA / cm 2 , 20mA / cm 2 、30mA / cm 2 , 40mA / cm 2 or 50mA / cm 2 ; The pH is 3, 4, 5, 6 or 7; and the reaction time is 0.5h, 1.0h, 1.5h, 2.0h, 2.5h or 3.0h.

[0060] In some embodiments, the method provided by S1 to S2 is adopted, and after the high-salt wastewater is subjected to an electrochemical reaction, the concentration of residual nitrate ions is less than 60 mg / L.

[0061] Accordingly, an embodiment of the present invention further provides a method for preparing a rare earth element doped cathode, which includes S11 to S13 described above. In the preparation method, by optimizing the ratio of the precursor solution, combined with repeated coating-high temperature sintering treatment and specific sintering conditions and crystallization treatment conditions, the selectivity and catalytic activity of the obtained rare earth element doped cathode for nitrate electrochemical reduction are further improved.

[0062] Correspondingly, an embodiment of the present invention also provides a rare earth element-doped cathode, which is prepared by the preparation method of the rare earth element-doped cathode. The rare earth element-doped cathode is a titanium-based oxide coating electrode doped with rare earth elements, and uses phosphorus and / or sulfur-containing inorganic salts or organic matter as additives to improve the selectivity of electrochemical reduction of nitrates to produce nitrogen and / or ammonia. It has high selectivity in the removal of nitrates from high-salt wastewater, and can remove nitrates by electroreducing them into nitrogen and ammonia.

[0063] In order to enable the above implementation details and operations of the present invention to be clearly understood by those skilled in the art, and to significantly demonstrate the improved performance of the method for electrochemical reduction of nitrates in high-salt wastewater in the embodiment of the present invention, the above technical scheme is illustrated by examples below.

[0064] The experimental methods in the following examples without specifying specific conditions are all conventional methods in the art. The materials used in the examples can be obtained from commercial sources unless otherwise specified.

[0065] Example

[0066] (1) Preparing a titanium-based oxide coating electrode by rare earth element doping includes:

[0067] 1) For 10*15cm2 The titanium plate electrode is surface pretreated by using 1% NaOH solution, acetone and deionized water to clean it in sequence and repeating it for 3 times or more; then it is graded and polished with coarse sandpaper, fine sandpaper, etc. until the oxide layer is completely removed; then it is immersed in 5%-10% oxalic acid solution for 12-48 hours.

[0068] 2) preparing a precursor solution containing metal oxides, rare earth elements and additives, wherein the molar concentration ratio of the metal oxides, rare earth elements and additives is 100:0.1:0.02, wherein the metal oxide is Sb-SnO2; the salt containing the rare earth element is cerium chloride; and the additive (sulfur and / or phosphorus-containing inorganic salt or organic matter) is sodium phosphosulfate.

[0069] 3) Layer-coat the prepared precursor solution on the electrode surface.

[0070] 4) The electrode coated with the precursor solution is subjected to high temperature sintering, wherein the temperature range of the high temperature sintering is 650° C., the pressure range is 0.5 MPa, and the sintering is performed in an oxygen-rich atmosphere for 50 minutes;

[0071] 5) Repeat coating and sintering the sintered electrode for 15 times;

[0072] 6) The electrode that has been repeatedly coated and sintered is subjected to constant temperature crystallization treatment, with the temperature controlled at 500° C. and the time controlled at 24 hours, thereby obtaining a rare earth element-doped cathode.

[0073] (2) The NaCl-containing brine produced by membrane salt separation and concentration of coal chemical wastewater is used as the high-salt wastewater, which contains 210 g / L NaCl and about 1065 mg / L nitrate. The amount of high-salt wastewater used in this embodiment is about 900 ml.

[0074] (3) The rare earth element doped cathode prepared in step (1) is used for electrochemical reduction to remove nitrates from high-salinity wastewater. In the electrolytic cell, a pair of electrodes is used, wherein the cathode is a coating electrode of titanium-based Sb-SnO2 oxide doped with rare earth elements and containing additives prepared in step (1), and the anode is a coating electrode of titanium-based Sb-SnO2 oxide with the same area but without rare earth element doping and additives. The applied current density is controlled to be 25 mA / cm 2 , the pH value of high-salt wastewater is 6.5, and the electro-reduction treatment time lasts about 3 hours.

[0075] The results show that the use of the prepared rare earth element doped cathode for electrochemical reduction to remove nitrates in high-salinity wastewater can reduce nitrates in wastewater mainly to N2 and a very small amount of ammonia, thereby efficiently removing nitrates from coal chemical concentrated brine. At the end of the experiment, the residual nitrate ion concentration in the treated high-salinity wastewater was less than 60 mg / L.

[0076] Example 2

[0077] The steps adopted in this embodiment are basically the same as those in Example 1, except that the cathode adopts a titanium-based Sb-SnO2 oxide coating electrode doped with rare earth elements and using additives, while the anode adopts a platinum sheet electrode of similar area. This group of electrodes is used for electrochemical reduction to remove nitrates in high-salinity wastewater, and is also controlled under the same applied potential and solution pH. At the end of the experiment, the residual nitrate ion concentration in the treated high-salinity wastewater was about 39 mg / L. At the same time, it was found that the main products of nitrate electroreduction using this group of electrodes were N2 and ammonia products. The results show that the effect of electrochemical reduction to remove nitrates in high-salinity wastewater mainly depends on the properties of the prepared rare earth element-doped titanium-based oxide coating cathode. When the anode uses a platinum electrode, the electrochemical reduction removal effect of nitrates is better, which is speculated to be due to the better conductivity of the platinum sheet electrode than the ordinary titanium-based oxide coating electrode.

[0078] Example 3

[0079] The steps adopted in this embodiment are basically the same as those in Example 1, except that two cathodes are used, one is a titanium-based Sb-SnO2 oxide-coated electrode doped with rare earth elements and using phosphorus-containing additives, and the other is a titanium-based Sb-SnO2 oxide-coated electrode doped with rare earth elements and using sulfur-containing additives; and the anode uses a titanium-based Sb-SnO2 oxide-coated electrode that is not doped with rare earth elements and does not contain additives. These two groups of electrodes (i.e., one group of cathodes uses phosphorus-containing additives and the other group of cathodes uses sulfur-containing additives) are used for electrochemical reduction to remove nitrates from high-salinity wastewater, and are also controlled at the same applied potential and solution pH. At the end of the experiment, the residual nitrate ion concentration in the treated high-salinity wastewater was less than 60 mg / L, and when the cathode used phosphorus-containing additives, the nitrate electroreduction products were almost all N2, and almost no ammonia products were detected; when the cathode used sulfur-containing additives, in addition to N2, ammonia reduction products were also detected. The results show that when preparing rare earth element-doped cathodes, phosphorus- and sulfur-containing additives can affect the selectivity of nitrate electro-reduction reactions, which means that by changing the type and properties of additives, the products of nitrate electro-reduction in high-salt wastewater can be controlled, which is also beneficial to promote the efficient removal of nitrates in high-salt wastewater.

[0080] Comparative Example 1

[0081] The steps used in this comparative example are basically the same as those in Example 1, except that titanium-based Sb-SnO2 oxide-coated electrodes without rare earth element doping and additives are used in both the cathode and the anode. This set of electrodes is used for electrochemical reduction to remove nitrates from high-salinity wastewater, and is also controlled under the same applied potential and solution pH. At the end of the experiment, the residual nitrate ion concentration in the treated high-salinity wastewater was about 380 mg / L. At the same time, it was found that the electrochemical reduction products of nitrates using this set of electrodes mostly produced NO2 in addition to a small amount of N2 and ammonia products. - and nitrate reduction intermediates such as NO, N2O, and NH2OH. This indicates that the use of titanium-based oxide coating electrodes that have not been doped with rare earth elements for electrochemical reduction and removal of nitrates in high-salinity wastewater not only has a low current efficiency, but also generates a variety of nitrate reduction intermediates, making it impossible to achieve efficient removal of nitrates in high-salinity wastewater.

[0082] Comparative Example 2

[0083] The steps used in this comparative example are basically the same as those in Example 1, except that the anode uses a titanium-based Sb-SnO2 oxide-coated electrode that is not doped with rare earth elements and does not contain additives, while the cathode uses a titanium-based Sb-SnO2 oxide-coated electrode that contains rare earth elements but does not use additives. This set of electrodes is used for electrochemical reduction to remove nitrates from high-salinity wastewater, and is also controlled under the same applied potential and solution pH. At the end of the experiment, the residual nitrate ion concentration in the treated high-salinity wastewater was about 95 mg / L. At the same time, it was found that the electrochemical reduction products of nitrates using this set of electrodes produced a large amount of NO2 in addition to a small amount of N2 and ammonia products. - and nitrate reduction intermediates such as NO, N2O, and NH2OH. This indicates that the titanium-based oxide coating electrode doped with rare earth elements but without additives can be used for electrochemical reduction to remove nitrate from high-salinity wastewater. Although it can significantly improve the current efficiency of nitrate electroreduction, the types of nitrate reduction intermediates generated are more numerous, and it is also impossible to achieve selective and efficient removal of nitrate from high-salinity wastewater.

[0084] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for removing nitrate from high-salinity wastewater by electrochemical reduction, characterized in that: The steps include: providing an anode and a rare earth element doped cathode; Adding high-salt wastewater into the electrolytic cell, and performing an electrochemical reduction reaction with the anode and the rare earth element-doped cathode to reduce the nitrate into nitrogen and / or ammonia for removal; Wherein, the preparation method of the rare earth element doped cathode comprises: Providing a titanium-based metal electrode and a precursor solution, wherein the precursor solution includes a metal oxide, a salt containing a rare earth element, and an inorganic salt or organic matter containing sulfur and / or phosphorus; Applying the precursor solution on the surface of the titanium-based metal electrode and performing a sintering treatment; Then, the cathode doped with rare earth elements is obtained by constant temperature crystallization treatment; The metal oxide is selected from at least one of Sb-SnO2, RuO2, and IrO2; the salt containing rare earth elements is chloride, sulfate and / or nitrate of La, Ce and / or Pr; The sulfur- and / or phosphorus-containing inorganic salt or organic matter is sodium sulfate or sodium phosphate; In the precursor solution, the molar concentration ratio of the metal oxide, the salt containing rare earth elements, and the sulfur- and / or phosphorus-containing inorganic salt or organic matter is 100:(0.05-1):(0.01-0.1); The high-salt wastewater is the concentrated brine produced by membrane salt separation and concentration of coal chemical wastewater, the centrifugal mother liquor of NaCl industrial salt removed by evaporation, concentration and crystallization, and the product after organic matter, calcium and magnesium ions, heavy metals and Si impurities are removed.

2. The method according to claim 1, characterized in that The sintering process is carried out in an oxygen-rich atmosphere at a pressure of 0.2-2.0 MPa, with a sintering temperature of 500-1200° C. and a sintering time of 30-120 min.

3. The method according to claim 1, characterized in that The temperature of the constant temperature crystallization treatment is 500-800° C. and the time is 12-48 hours.

4. The method according to claim 1, characterized in that: During the electrochemical reduction reaction, the applied current density is 10-50 mA / cm 2 The pH of the high-salt wastewater is controlled at 3-7, and the reaction time is 0.5-3h.

5. The method according to any one of claims 1 to 4, characterized in that: The steps of coating the precursor solution on the surface of the titanium-based metal electrode and performing sintering treatment are repeated multiple times.

6. The method according to claim 5, characterized in that The sintering process is repeated 10-20 times.

7. The method according to any one of claims 1 to 4, characterized in that: The titanium-based metal electrode is also subjected to surface pretreatment.

8. The method according to claim 7, characterized in that The surface pretreatment includes at least one of removing oil, removing oxidation layer, and forming a rough surface.

9. The method according to claim 8, characterized in that The degreasing step is to sequentially clean the titanium-based metal electrode with alkaline solution, organic solvent and deionized water, and repeat the process at least three times; the oxidation layer removal step is to gradedly grind with coarse sandpaper and fine sandpaper until all the oxidation layers on the surface of the titanium-based metal electrode are removed; the rough surface formation step is to soak the titanium-based metal electrode in a 5%-10% oxalic acid solution for 12-48 hours.

10. The method according to any one of claims 1 to 4, characterized in that: After the high-salinity wastewater undergoes electrochemical reaction, the concentration of residual nitrate ions is lower than 60 mg / L.

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