Method for reducing nitrate based on anode and cathode conversion

Abstract

The application discloses a method for realizing nitrate reduction based on anode and cathode conversion and belongs to the technical field of water pollution treatment. The method uses a transition double-metal oxide electrode as the anode and the cathode of a reduction system, and realizes a high-adsorption-energy, high-electron-transmission-performance and high-efficiency denitrification technology by regulating and controlling the low-frequency conversion of the anode and the cathode.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of water pollution treatment, and particularly relates to a method for realizing nitrate reduction based on anode and cathode conversion. BACKGROUND

[0002] In recent years, nitrogen pollution in water environment has become a global environmental problem. Excessive nitrate content in water not only deteriorates the water environment, but also seriously affects the life activities of aquatic organisms and poses a risk to human health. In particular, in some special industries, high-concentration nitrate in waste is one of the key problems in waste treatment. The nitrate in waste will greatly increase the volume of waste and also have a negative impact on the stability of subsequent solidification treatment.

[0003] Currently, the main methods for treating nitrate in wastewater include ion exchange, reverse osmosis, biological denitrification, catalytic hydrogenation, and electrochemical reduction. Among them, the biological method has a long denitrification period, requires additional carbon source, and the microbial activity is greatly affected by water quality pH value and temperature, etc., which has problems such as high treatment cost and complex operation and management; ion exchange method and reverse osmosis method have high operation cost, and the concentrated liquid needs to be treated again; compared with the above methods, electrochemical reduction method has the advantages of low investment cost, environmental friendliness, and high efficiency, and has a good application prospect for the denitrification treatment of nitrate in wastewater.

[0004] From the aspects of reduction activity and reduction rate, the current technology applied to electrochemical reduction of nitrate still has some defects:

[0005] (1) Although noble metal cathode materials such as Pd have high activity and nitrogen selectivity for nitrate reduction, from the economic point of view, due to the high cost and scarcity of noble metals, the industrial application progress is restricted. The activity and nitrogen selectivity of existing non-noble metal active electrodes in the chemical reduction process of nitrate still need to be further improved.

[0006] (2) The essence of electrochemical nitrate reduction is a surface interface reaction, which occurs in the double electric layer on the surface of the cathode. That is, only when the electroactive substance in the solution is adsorbed to the active site on the surface of the cathode material, the reaction will occur. However, the adsorption energy of the metal cathode materials widely studied at present for nitrate is weak, which makes the removal rate of the electrocatalytic reaction system of nitrate easily affected by the mass transfer diffusion rate. It is of great significance to find a reduction technology with high adsorption performance for the degradation of nitrate. The metal cathode materials widely studied at present, which are based on conductive metal substrates (such as titanium substrate), have the defect that the adsorption energy of the metal cathode materials for nitrate is weak, and the removal rate is easily affected by the mass transfer diffusion rate. Based on this, the cathode materials at present are mostly coating catalysts synthesized based on foam nickel and carbon felt as substrates, but such electrodes have problems such as complex operation process, high preparation cost, and poor mechanical properties.

[0007] Therefore, from the overall point of view, it is of great significance to obtain an industrial application of low cost and strong comprehensive reduction of nitrate, and to remove nitrate in water bodies to achieve green denitrification. SUMMARY

[0008] 1. Problem to be solved

[0009] Based on the above problems of electrochemical reduction of nitrate based on non-noble metal active electrode, the application provides a method for realizing reduction of nitrate based on anode and cathode conversion.

[0010] 2. Technical scheme

[0011] In order to solve the above problems, the technical scheme adopted by the application is as follows:

[0012] The first aspect of the application provides a method for realizing reduction of nitrate based on anode and cathode conversion, comprising:

[0013] Contacting electrode one and electrode two with the water body to be treated;

[0014] Converting the anode and cathode polarity of electrode one and electrode two at regular intervals;

[0015] The conversion frequency is 1-5 min / time;

[0016] The water body to be treated contains nitrate;

[0017] The electrode one contains transition metal oxide, and the electrode two contains transition metal oxide;

[0018] The electrode one and the electrode two work under direct current power and constant current conditions.

[0019] According to any embodiment of the first aspect of the application, before the conversion is performed, the power is turned off and maintained for 1-3 s.

[0020] According to any embodiment of the first aspect of the application, the current density is 10-20 mA / cm 2 .

[0021] According to any embodiment of the first aspect of the application, the distance between the electrode one and the electrode two is 5-50 mm.

[0022] It should be noted that the electrode distance is within a certain range, and the smaller the electrode distance, the higher the current efficiency, but it cannot be too small or too large. Specifically, too small electrode distance will cause electrode breakdown, and too large electrode distance will result in low current efficiency. Based on this, from the perspective of energy consumption, the distance between the electrode one and the electrode two is controlled to be 5-50 mm.

[0023] According to any embodiment of the first aspect of the present application, the nitrate content in the water body to be treated is 10-1000 mg / L.

[0024] According to any embodiment of the first aspect of the present application, the water body to be treated also contains electrolytes, and the concentration of the electrolytes is controlled at 0.01-1 mol / L.

[0025] The electrolyte as described herein mainly functions to reduce solution resistance and promote electron transfer. Based on this, the concentration of the electrolyte will cause changes in solution resistance, specifically, too high a concentration will cause excessive sodium ions to compete with nitrate nitrogen for adsorption, and too low a concentration will affect the conductivity of the solution, thereby increasing the solution resistance. Based on this, from the perspective of solution resistance, the concentration of the electrolyte is not less than 0.01 mol / L, but it is necessary to clarify that it is not more than the concentration at which it reaches saturation.

[0026] Further, the electrolyte can be exemplified by, for example, sodium sulfate, potassium sulfate, sodium hydroxide, and potassium hydroxide.

[0027] According to any embodiment of the first aspect of the present application, the electrode one contains oxides of two or more kinds of transition metals.

[0028] According to any embodiment of the first aspect of the present application, the electrode two contains oxides of two or more kinds of transition metals.

[0029] According to any embodiment of the first aspect of the present application, the transition metals include, but are not limited to, chromium, manganese, iron, cobalt, nickel, copper, and zinc.

[0030] According to any embodiment of the first aspect of the present application, the electrode one has a conductive metal as a substrate, and the substrate is loaded with transition metal oxides.

[0031] It is necessary to clarify here that the transition metal oxides on the substrate serve as active metals for electrochemical reduction of nitrate nitrogen, and the loading amount will affect the degradation effect. Based on this, the loading amount of the transition metal oxides on the substrate, in terms of transition metals, is not less than 0.1 g / cm 2 .

[0032] According to any embodiment of the first aspect of the present application, the electrode two has a conductive metal as a substrate, and the substrate is loaded with transition metal oxides.

[0033] The electrode one and the electrode two as described herein are each based on an electrically conductive metal, and a transition metal oxide is loaded on the base. On this basis, the base can be mentioned, for example, a Ti base (such as a titanium sheet, a titanium mesh, a titanium plate), a foamed nickel, a carbon base (carbon felt, carbon cloth, carbon brush, biochar), and the like.

[0034] Further, the electrode one and the electrode two can be completely the same, that is, the base has the same kind, and the transition metal loaded on the base has the same kind;

[0035] The electrode one and the electrode two can also be of different kinds, for example:

[0036] The electrode one and the electrode two have the same kind of base, but different kinds of transition metals are loaded on the base;

[0037] The electrode one and the electrode two have different kinds of base, but the same kind of transition metal is loaded on the base;

[0038] The electrode one and the electrode two have different kinds of base, and different kinds of transition metal are loaded on the base.

[0039] Further, the loading of the transition metal on the base and the formation of the electrode one or the electrode two can be realized by using, for example, a sol-gel-calcination method, an electrodeposition method, a hydrothermal method, and the like.

[0040] According to any embodiment of the first aspect of the purpose of the present application, an electrically conductive metal is used as a base, and a sol-gel-calcination method is used to treat the base to form the electrode one and the electrode two.

[0041] Advantages

[0042] In view of the defects that the metal cathode material based on an electrically conductive metal currently widely studied has weak adsorption ability for nitrate, and the removal rate is easily affected by the mass transfer diffusion rate, the present application proposes a method for improving the adsorption ability of nitrate based on the conversion between the anode and the cathode and reducing nitrate, specifically:

[0043] (1) In the method, the nitrate is first adsorbed by the anode through heterogeneous phase adsorption, and the transition metal cation with high valence on the surface of the original anode will continuously adsorb the negatively charged nitrate after the electrode is reversed, and the adsorbed nitrate will be rapidly reduced. This technology has high nitrate adsorption, and solves the problem that the existing cathode has weak adsorption ability for nitrate.

[0044] (2) The method, the cathode and the anode both use the electrode with double transition metals, and the conversion of the electrode can be combined to simultaneously perform the anode adsorption and the cathode reduction, the electron transmission performance is rich, the electrochemical activity is high, and the method is more efficient than the traditional electrocatalytic reduction of nitrate.

[0045] (3) In the method, before the polarity conversion of the cathode and the anode is performed each time, the power supply is turned off first, and then maintained for 1-3 s, so that the transition metal ions on the anode can be maintained in a transient high valence state, the continuous adsorption of the nitrate with negative electricity can be realized, and the optimization of the adsorption effect can be ensured. After the polarity conversion of the cathode and the anode is performed, the transition metal ions on the original anode directly change to a normal valence state, and the subsequent reduction efficiency is reduced.

[0046] (4) The main product of the reduction of nitrate in the method is nitrogen, no secondary pollution is generated, the operation is simple, the occupied area is small, and the industrialization is easy to realize. The method is a high-efficiency, green denitrification and environment-friendly nitrate nitrogen removal method. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The low-frequency conversion technology mechanism involved in Example 1 of the application is shown in the figure Figure 1 ;

[0048] Figure 2 The low-frequency conversion technology mechanism involved in Example 1 of the application is shown in the figure Figure 2 ;

[0049] Figure 3 The low-frequency conversion technology mechanism involved in Example 1 of the application is shown in the figure Figure 3 ;

[0050] Figure 4 The surface SEM image of the titanium plate after pretreatment in the electrode preparation process used in Example 1 of the application is shown in the figure

[0051] Figure 5 The surface SEM image of the electrode used in Example 1 of the application is shown in the figure

[0052] Figure 6 The degradation effect trend graph of electrochemical reduction of nitrate in Example 1 and Comparative Example 1 of the application is shown in the figure

[0053] Figure 7-1 The degradation effect trend graph of electrochemical reduction of nitrate in Example 1, Example 2 and Example 3 of the application is shown in the figure

[0054] Figure 7-2 The degradation effect trend graph of electrochemical reduction of nitrate in Comparative Example 2, Comparative Example 3 and Comparative Example 4 of the application is shown in the figure

[0055] Figure 8-1 Trend graph of degradation effect of electrochemical reduction of nitrate salt for Example 1, Example 4 and Example 5 of the present invention;

[0056] Figure 8-2 Trend graph of degradation effect of electrochemical reduction of nitrate salt for Comparative Example 5 and Comparative Example 6 of the present invention;

[0057] Figure 9 Comparison graph of degradation effect for Specific Example 1, Example 6 and Comparative Example 7 of the present invention. DETAILED DESCRIPTION

[0058] The present disclosure can be more easily understood and further advantages and benefits can be obtained by reference to the following description and examples in conjunction with the accompanying drawings in which: it should be understood that the present disclosure is not limited to the specific products, methods, conditions or parameters described and / or shown herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting, unless otherwise specified.

[0059] It should also be understood that, for clarity, certain features of the present disclosure can be described in the context of separate embodiments in this document, but can also be provided in combination with each other in a single embodiment. That is, unless explicitly incompatible or specifically excluded, each separate embodiment is considered to be combinable with any other embodiment, and such combinations are considered to represent another, different embodiment. Conversely, various features of the present disclosure described in the context of a single embodiment can also be provided separately or in any subcombination, for the sake of brevity. Finally, while a particular embodiment can be described as part of a series of steps or part of a more general structure, each step or substructure can itself be considered an independent embodiment.

[0060] Unless otherwise stated, it is to be understood that each individual element in a list and each combination of individual elements in that list is to be interpreted as a separate embodiment. For example, a list of embodiments recited as“A, B, or C” is to be interpreted as including the embodiments“A,”“B,”“C,”“A or B,”“A or C,”“B or C,” or“A, B, or C.”

[0061] In this document, the singular forms“a,”“an,” and“the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to“a substance” is a reference to at least one of such a substance and equivalents thereof.

[0062] Terms including ordinal numbers such as "first" and "second" can be used to explain various components or fluids, but the components, fluids are not limited by the terms. Therefore, the terms are used only to distinguish the component / fluid from another component / fluid without departing from the teachings of the present disclosure.

[0063] When items are described using conjunctive terms such as "and / or", the description should be understood to include any one of the associated listed items and all combinations of one or more of them.

[0064] In general, the use of the term "about" indicates an approximation that can vary depending on the desired characteristics obtained by the disclosed subject matter and will be interpreted based on functionality in a context-dependent manner. Therefore, a person of ordinary skill in the art will be able to interpret the degree of difference on a case-by-case basis. In some cases, the number of significant digits used in expressing a particular value can be representative of the degree of precision to which the term "about" allows for variation. In other cases, a range of values in a series of values can be used to determine the range of variation allowed by the term "about". Further, all ranges in the present disclosure are inclusive and combinable, and the mention of a value stated in a range includes every value within the range.

[0065] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs; the terms used herein and / or any and all combinations of one or more related listed items.

[0066] The present application provides a method for reducing nitrate based on the conversion of cathode and anode, the steps are as follows:

[0067] S1. Contacting electrode one, electrode two with the water body to be treated;

[0068] S2. The cathode and anode of the electrode one and electrode two are converted at a frequency of 1-5 min / time; and preferably, before the conversion is performed, the power supply is turned off and maintained for 1-3 s.

[0069] In addition, the electrode one and electrode two work under direct current power supply and constant current condition, and the current density during work can be 10-20 mA / cm 2 , and the distance between electrode one and electrode two is 5-50 mm.

[0070] The present application provides a method for reducing nitrate based on the conversion of cathode and anode, the principle is as follows:

[0071] Based on the timing conversion of cathode and anode, the transition metal on electrode one and electrode two can be controlled to be always in the state of phase conversion between low valence and high valence. The main mechanism of the technology is further described as follows: (1) before the cathode and anode conversion of electrode one and electrode two, the transition metal ions on the anode are oxidized to a transient high valence, and the nitrate ions are adsorbed on the electrode surface by the hetero-phase attraction of the anode (referring to the state shown in Figure 1 ); (2) before the cathode and anode conversion of electrode one and electrode two each time, the power is turned off first and kept for 1-3s, so that the transition metal ions on the original anode can be maintained in a transient high valence, preventing the transition metal ions from directly changing to a normal valence after the cathode and anode conversion, and the repulsion between the cathode and the nitrate, reducing the subsequent reduction efficiency (referring to the state shown in Figure 2 ); (3) after the power is turned on and the cathode and anode of electrode one and electrode two are converted, the transient high valence metal ions on the surface of the new anode (original cathode) continue to adsorb the nitrate, and after the transition to a normal valence, the nitrate adsorbed on the surface is reduced as an active metal (referring to the state shown in Figure 3 ).

[0072] The method for reducing nitrate based on the conversion between cathode and anode provided by the application can use practical electrodes prepared by existing technologies such as sol-gel-calcination method, electrodeposition method, and hydrothermal method.

[0073] For example, the sol-gel-calcination method is as follows: a mixed solution containing transition metal ions (generally transition metal salts) is prepared as a precursor solution, the precursor solution is coated on a conductive metal substrate, then drying treatment is performed, the coating and drying operations are repeated for several times, and finally high-temperature calcination treatment is performed, so that the electrode is obtained.

[0074] For reference, the concentration of transition metal in the precursor solution is controlled to be 0.5-2mol / L.

[0075] For reference, the number of repetitions of the coating is controlled to be 6-12 times.

[0076] For reference, the high-temperature calcination temperature is controlled to be 300-600℃, and the calcination time is controlled to be 1-2h.

[0077] For reference, the temperature of the high-temperature calcination is controlled to be 300-600℃, and the temperature rising speed is controlled to be 5℃ / min.

[0078] The electrodeposition method is as follows: a mixed solution containing transition metal ions (generally transition metal salts), boric acid and ionic surface exchange agent is prepared as an electrodeposition solution, Ag / AgCl is used as a reference electrode, a pretreated substrate is used as a cathode, platinum or graphite is used as an anode, electrodeposition is carried out under constant current for a period of time, the electrodeposition is repeated several times, drying is carried out, and finally high-temperature calcination is carried out, so that the electrode is obtained.

[0079] The concentration of the transition metal in the electrodeposition solution is controlled at 0.1-1 mol / L.

[0080] The ionic surface exchange agent in the electrodeposition solution is sodium eicosyl sulfonate.

[0081] The concentrations of the boric acid and the surface exchange agent in the electrodeposition solution are controlled at 0.1-1 mol / L and 0.2-2 g / L, respectively.

[0082] The number of repetitions of the coating is controlled at 2-20 times.

[0083] The electrodeposition time is controlled at 1-10 min.

[0084] The temperature of the high-temperature calcination is controlled at 300-600 ℃, and the temperature rising speed is controlled at 5 ℃ / min.

[0085] The hydrothermal method is as follows: a mixed solution containing transition metal ions (generally transition metal salts), urea and ammonium fluoride is prepared as a precursor solution, the precursor solution and a substrate are placed in a hydrothermal kettle for hydrothermal reaction, washing and drying are carried out after the hydrothermal reaction, and finally high-temperature calcination is carried out, so that the electrode is obtained.

[0086] The concentration of the transition metal in the precursor solution is controlled at 0.001-0.1 mol / L.

[0087] The concentrations of the urea and the ammonium fluoride in the precursor solution are controlled at 5-20 g / L and 10-30 g / L, respectively.

[0088] The hydrothermal time is controlled at 3-6 h, and the hydrothermal temperature is controlled at 120-200 ℃.

[0089] The temperature of the high-temperature calcination is controlled at 300-600 ℃, and the temperature rising speed is controlled at 5 ℃ / min.

[0090] It should be noted that, before the preparation process, the substrate is generally pretreated.

[0091] The pre-treatment operation that can be referenced when the substrate is a titanium substrate is: polishing-alkali etching-acid etching of the titanium substrate. Specifically: prepare a titanium plate of a certain size, polish the titanium plate with coarse sandpaper and fine sandpaper, then place it in an alkali solution of a certain concentration and in a 40-70°C water bath for a certain time to remove surface grease, then in an acid solution of a certain concentration and in a 70-90°C water bath for a certain time to increase the anchoring sites on the surface, and then place the material in deionized water for ultrasonic immersion for 10-20 min.

[0092] The model of the coarse sandpaper and the fine sandpaper can be 120-240 mesh and 400-800 mesh, respectively.

[0093] The acid solution can include any one, two or more of oxalic acid, sulfuric acid and hydrochloric acid.

[0094] The concentration of the acid and alkali added in the acid solution and the alkali solution can be 10-30 wt%, and on this basis, the acid etching and alkali etching time is controlled to be 1-2 h.

[0095] The pre-treatment operation that can be referenced when the substrate is a nickel foam is: deoxidation, impurity removal and the like of the nickel foam. Specifically: prepare a nickel foam of a certain size, first immerse it in an acetone solution for a period of time for ultrasonic removal of organic matter on the surface, then rinse it with deionized water, then continue to immerse it in a sulfuric acid solution for a period of time for ultrasonic removal of oxides on the surface, then rinse it with deionized water, then immerse it in anhydrous ethanol for a period of time, and then place the treated nickel foam in anhydrous ethanol for use.

[0096] The acetone ultrasonic time can be controlled to be 10-30 min.

[0097] The concentration of the sulfuric acid solution used can be 1 mol / L, and the sulfuric acid ultrasonic time can be controlled to be 10-30 min.

[0098] The anhydrous ethanol ultrasonic time can be controlled to be 10-30 min.

[0099] The pre-treatment operation that can be referenced when the substrate is a carbon substrate is consistent with the pre-treatment operation of the nickel foam.

[0100] The following embodiments are specifically described with the titanium plate as the conductive metal substrate, and the same electrodes I and II prepared by the sol-gel-calcination method are used to make a more detailed description of the technical scheme of the application.

[0101] Step 1: polish the titanium plate of a size of 3x4 cm 2The titanium plate was polished until smooth, then placed in a 20wt% NaOH solution at 50°C for 1.5 hours. After removal, the surface of the titanium plate was repeatedly rinsed with deionized water until it became neutral (pH=7). Then, it was placed in a 20wt% oxalic acid solution at 80°C for 1.5 hours, and finally immersed in deionized water and sonicated for 5 minutes to complete the pretreatment. Figure 4 The image shows a SEM image of the pretreated titanium plate. It can be seen from the image that the surface of the pretreated titanium plate is uniformly rough and has a porous structure, which can increase the number of anchorable positioning points on the surface of the electrode later.

[0102] Step 2: Prepare a mixed solution of 1 mol / L cobalt nitrate and 1 mol / L ferric nitrate as a precursor solution. Use degreased cotton to evenly brush the precursor solution onto the Ti substrate pretreated in Step 1, and then place it in an oven at 105℃ to dry for 10 min. Repeat the above steps 10 times.

[0103] Step 3: Place the brush-coated Ti substrate into a muffle furnace and calcine at 500℃ for 1 hour at a heating rate of 5℃ / min. After calcination, allow it to cool naturally to room temperature to obtain a dual transition metal composite electrode containing cobalt and iron (existing as metal oxides). Clean the electrode surface with deionized water for later use. Figure 5 The image shows a surface SEM image of the dual transition metal composite electrode. The image shows that the loaded active material, namely the dual transition metal oxide, is uniformly distributed on the electrode surface, proving that the electrode was successfully prepared.

[0104] The present invention will be further illustrated below with reference to specific embodiments, but these embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field. The essential features and significant effects of the present invention can be seen from the following embodiments. The described embodiments are some, but not all, embodiments of the present invention, and therefore do not limit the present invention in any way. Any non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are within the protection scope of the present invention.

[0105] Example 1

[0106] Electrochemical reduction of nitrate steps:

[0107] The dual transition metal composite electrode prepared above was used as the electroreduction cathode and anode for nitrate reduction, respectively.

[0108] A mixed solution of 100 mg / L NO3-N and 0.05 M sodium sulfate was used as the electrolyte, and it was placed in a container as follows: Figures 1 to 3 In the electrochemical reactor shown, a two-electrode system without an ion-exchange membrane is used, where the distance between the anode and cathode is 2 cm and the constant current density is 20 mA / cm².2 .

[0109] The cathode and anode in step two were switched at low frequency, and the cathode and anode were switched at a frequency of 1 min / time after the reaction started. The power was turned off for 1 s before each switching to keep the anode metal in a high valence state, and the reaction was carried out for 2 h.

[0110] As shown in Figure 1 , the Fe 2+ / 3+ and Co 2+ / 3+ on the anode were oxidized to a transient high valence state, i.e., Fe 6+ and Co 5+ , before the cathode and anode were switched, and the nitrate was adsorbed on the electrode surface by heterogeneous-phase adsorption of the anode;

[0111] As shown in Figure 2 , the power was turned off for 1 s before the cathode and anode were switched to keep the Fe 6+ and Co 5+ on the anode in a transient high valence state;

[0112] As shown in Figure 3 , then, the power was turned on again and the cathode and anode were switched. The Fe 6+ and Co 5+ on the surface of the cathode after the switching (the anode before the switching) continued to adsorb the nitrate, and after the Fe 2+ / 3+ and Co 2+ / 3+ were converted to normal valence states, they reduced the adsorbed nitrate on the surface.

[0113] Comparative Example 1

[0114] This comparative example is basically the same as Example 1, except that the cathode and anode were not switched during the electrochemical reduction of the nitrate.

[0115] Figure 6 This is a comparison chart of the nitrate reduction and degradation effects of Example 1 and Comparative Example 1 of the present application. As can be seen from the chart:

[0116] Switching the cathode and anode significantly improves the reduction effect of the nitrate. In Example 1 of the present application, the degradation rate reached 100% at 100 min using the low-frequency switching technology of the cathode and anode.

[0117] The degradation rate of Comparative Example 1, which did not use the cathode and anode switching technology, was only 66.87% at the same reaction time, which was 33.13% lower than that of Example 1, indicating that switching the cathode and anode during the electrochemical reduction of the nitrate is more efficient and has stronger reduction capacity than the ordinary constant-current reduction method.

[0118] Example 2

[0119] This example is substantially the same as Example 1, except that the cathode and anode are switched at a frequency of 2 minutes per switch after the reaction is initiated during the electrochemical reduction of the nitrate salt.

[0120] Example 3

[0121] This example is substantially the same as Example 1, except that the cathode and anode are switched at a frequency of 5 minutes per switch after the reaction is initiated during the electrochemical reduction of the nitrate salt.

[0122] Comparative Example 2

[0123] This comparative example is substantially the same as Example 1, except that the cathode and anode are switched at a frequency of 10 seconds per switch after the reaction is initiated during the electrochemical reduction of the nitrate salt.

[0124] Comparative Example 3

[0125] This comparative example is substantially the same as Example 1, except that the cathode and anode are switched at a frequency of 30 seconds per switch after the reaction is initiated during the electrochemical reduction of the nitrate salt.

[0126] Comparative Example 4

[0127] This comparative example is substantially the same as Example 1, except that the cathode and anode are switched at a frequency of 8 minutes per switch after the reaction is initiated during the electrochemical reduction of the nitrate salt.

[0128] Figure 7-1 A comparison chart of the nitrate salt reduction and degradation effects of Examples 1, 2, and 3 of the present application. As can be seen from the chart:

[0129] When the cathode and anode switching frequency is 1 minute per switch, the degradation rate is the highest, reaching 100% degradation in 120 minutes;

[0130] And when the cathode and anode switching frequency is 2 minutes per switch, the degradation rate is 90.32%;

[0131] When the cathode and anode switching frequency is 5 minutes per switch, the degradation rate is 82.32%;

[0132] As can be seen, when the cathode and anode switching frequency is 2 minutes per switch or 5 minutes per switch, the degradation rate is slightly lower than when the cathode and anode switching frequency is 1 minute per switch; but overall, when the cathode and anode switching frequency is 1-5 minutes per switch, the degradation rate of the nitrate salt in 120 minutes is higher than 80%, which is at a relatively high level.

[0133] Figure 7-2 A comparison chart of the nitrate salt reduction and degradation effects of Comparative Examples 2, 3, and 4 of the present application. As can be seen from the chart:

[0134] When the anode and cathode conversion frequency is 10 s / time, the degradation rate at 120 min is 61.19%, which is 38.81% lower than the degradation rate when the anode and cathode conversion frequency is 1 min / time;

[0135] When the anode and cathode conversion frequency is 30 s / time, the degradation rate at 120 min is 66.53%, which is 33.47% lower than the degradation rate when the anode and cathode conversion frequency is 1 min / time;

[0136] When the anode and cathode conversion frequency is 8 min / time, the degradation rate is 61.32%;

[0137] From the comparison of the above embodiment 1 and comparative examples 2, 3, 4, it can be seen that when the anode and cathode conversion frequency is too high or too low, the degradation effect is poor. When the anode and cathode conversion frequency is too high, the adsorption time of nitrate nitrogen by the anode is shortened, the adsorption force is weakened, and at the same time, the degradation of nitrate nitrogen by the cathode becomes an intermittent reaction, resulting in a low reduction efficiency. When the anode and cathode conversion frequency is too low, the metal ions of the anode cannot maintain a high valence state, and after being converted to the cathode, the adsorption capacity of the cathode for nitrate is reduced, resulting in a low degradation rate.

[0138] Example 4

[0139] This example is basically the same as example 1, and the only difference is that in the process of electrochemical reduction of nitrate, the power is turned off for 2 s before each conversion to keep the anode metal in a high valence state.

[0140] Example 5

[0141] This example is basically the same as example 1, and the only difference is that in the process of electrochemical reduction of nitrate, the power is turned off for 3 s before each conversion to keep the anode metal in a high valence state.

[0142] Comparative example 5

[0143] This comparative example is basically the same as example 1, and the only difference is that in the process of electrochemical reduction of nitrate, the power is not turned off before each conversion, but the anode and cathode are directly converted.

[0144] Comparative example 6

[0145] This comparative example is basically the same as example 1, and the only difference is that in the process of electrochemical reduction of nitrate, the power is turned off for 5 s before each conversion to keep the anode metal in a high valence state.

[0146] Figure 8-1 The figure is a comparison of the nitrate reduction and degradation effects of examples 1, 4 and 5 of the present application. As can be seen from the figure:

[0147] With the extension of the power-off time before the cathode and anode conversion, the degradation rate of nitrate gradually decreases, and when the power-off time is 1s, the degradation rate is 100%;

[0148] When the power-off time is extended to 2s and 3s, the degradation rates are 92.28% and 82.57% respectively, which are slightly lower than the degradation rate when the power-off time is 1s. However, it can be seen as a whole that when the power-off time is controlled within 1-3s, the degradation rate of nitrate at 120min is higher than 80%, which is at a high level.

[0149] Figure 8-2 The nitrate reduction degradation effect comparison chart of the present application comparative examples 5 and 6. As can be seen from the chart:

[0150] When the power is not paused during the cathode and anode conversion, the degradation rate at 120min is 74.29%, which is 25.71% lower than the degradation rate when the power is paused for 1s;

[0151] When the power is paused for 5s during the cathode and anode conversion, the degradation rate at 120min is 60.57%, which is 39.43% lower than the degradation rate when the power is paused for 1s;

[0152] From the above comparison of the present application examples 1, 4 and 5 and comparative examples 5 and 6, it can be seen that when the power-off time is too high or too low, the degradation effect is poor, which may be due to:

[0153] When the power is not paused or the power-off time is short during the cathode and anode conversion, the reduction effect of the cathode after the anode directly converts to the cathode makes the metal ions unable to maintain a high valence state, thereby reducing the adsorption of nitrate and leading to a decrease in removal rate;

[0154] When the power-off time is long before the cathode and anode conversion, the nitrate adsorbed near the anode is re-dissolved in water, and after the cathode and anode conversion when the power is restarted, there is no nitrate adsorbed as an anode near the cathode, so the treatment effect will decrease.

[0155] Example 6

[0156] This example is basically the same as example 1, the difference is that in this comparative example, the electrode initially used as an anode is replaced by a titanium-ruthenium-iridium sheet electrode, and the electrode initially used as a cathode remains unchanged.

[0157] Comparative example 7

[0158] This comparative example is basically the same as example 6, the difference is that the cathode and anode conversion is not performed during the electrochemical reduction of nitrate.

[0159] Figure 9 The degradation effect comparison chart of the present application examples 1, 6 and comparative example 7, as can be seen from the chart:

[0160] When the anode of the reduction system is replaced by titanium ruthenium iridium sheet electrode, the degradation rate decreases from 100% to 88.67%, which shows that when the cathode and anode use the same double transition metal composite electrode, the adsorption of the anode and the reduction of the cathode can be realized at the same time, which is conducive to the degradation of nitrate, and is superior to the traditional titanium ruthenium iridium sheet anode.

[0161] When the conversion of the cathode and anode is cancelled, the degradation rate is further reduced to 79.56%, and the degradation rate of nitrate in Example 6 is nearly 10% higher than that in Comparative Example 7, which shows that the technology is not only suitable for the system with the same cathode and anode, but also has beneficial effects when applied to the traditional titanium ruthenium iridium anode system.

[0162] The above is only the preferred embodiment of the present application, and it should be pointed out that for ordinary skilled persons in the art, without departing from the technical principles of the present application, a number of improvements and modifications can be made, and these improvements and modifications should be considered as the protection scope of the present application.

Claims

1. A method for nitrate reduction based on anodizing and anodic conversion, characterized in that, Make electrode one and electrode two come into contact with the water to be treated; The cathode and anode of electrode one and electrode two are switched periodically; The switching frequency is 1~5 min / time; Before the conversion, turn off the power and keep it off for 1 to 3 seconds; The water to be treated contains nitrate ions; The electrode one contains two or more types of transition metal oxides; The second electrode contains two or more types of transition metal oxides; The transition metals include chromium, manganese, iron, cobalt, nickel, and copper; Electrode one and electrode two operate under DC power supply and constant current conditions.

2. The method for nitrate reduction based on anodizing and cathode conversion according to claim 1, characterized in that, Current density is 10~20 mA / cm 2 .

3. The method for nitrate reduction based on anodizing and cathode conversion according to claim 1, characterized in that, The distance between electrode one and electrode two is 5~50 mm.

4. The method for nitrate reduction based on anodizing and cathode conversion according to any one of claims 1 to 3, characterized in that, The nitrate content in the water to be treated is 10~1000 mg / L.

5. The method for nitrate reduction based on anodizing and cathode conversion according to claim 4, characterized in that, The water to be treated also contains electrolytes, the concentration of which is controlled at 0.01~1 mol / L.

6. The method for nitrate reduction based on anodizing and cathode conversion according to claim 5, characterized in that, The electrode is based on a conductive metal substrate, and a transition metal oxide is loaded on the substrate. The second electrode is based on a conductive metal substrate, on which a transition metal oxide is loaded.

7. The method for nitrate reduction based on anodizing and cathode conversion according to claim 6, characterized in that, Using a conductive metal as a substrate, the substrate is treated with a sol-gel-calcination method to form electrode one and electrode two.

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