JP7279993B2 - Method for treating nitrogen oxides - Google Patents

Description

TECHNICAL FIELD The present invention relates to a treatment method for removing oxidized nitrogen components in water to be treated by electrolysis.

Methods for treating nitrate nitrogen contained in wastewater include biological treatment methods that utilize the denitrifying ability of microorganisms, physicochemical treatment methods such as ion exchange, reverse osmosis, and electrodialysis, and electrolysis. There is an electrochemical treatment method using

The biological treatment method is the most widely used method due to its low running cost, but its slow reaction rate requires large-scale treatment equipment to treat a large amount of wastewater. In addition, this biological treatment method is difficult to apply to wastewater containing nitrate nitrogen at a high concentration of about 1 g/L or more, and the load on the treatment equipment due to changes in the concentration of nitrate nitrogen in the wastewater. Fluctuations tend to destabilize processing performance.

The physicochemical treatment method is a method that can reduce the size of the treatment apparatus and can be expected to perform reliable treatment. However, since this method is a method of separating and concentrating nitrogen in water, it is necessary to separately treat the liquid in which nitrogen is finally concentrated, and the nitrogen is fundamentally not treated.

The electrochemical treatment method using electrolysis is a method of fundamentally treating nitrogen components, and the treatment capacity is relatively large compared to the size of the equipment, and it is applicable to wastewater containing high concentrations of nitrogen. It is possible, and stable treatment is expected against fluctuations in the load on the treatment equipment based on changes in nitrogen concentration and the like.

However, among the conventional electrochemical nitrogen treatment methods, especially with respect to the treatment of oxidized nitrogen such as nitrate and nitrite, the durability (corrosion resistance) of the cathode that reduces the nitrate and nitrite nitrogen is Another problem is that a diaphragm or the like is required to separate the anode and the cathode.

Therefore, the inventors of the present invention have found that the metal component constituting the electrode is a A nitrogen oxide treatment method capable of preventing elution into a solution and performing an effective and efficient electrolytic treatment is provided (Patent Document 1).

Specifically, an example in which nitrate nitrogen is decomposed is described in Examples of Patent Document 1, and it is shown that nitrate nitrogen is effectively decomposed.

However, it can be seen that the decomposition progressed and the decomposition rate decreased in the region where the nitrate nitrogen concentration was generally below 500 mg/L. This is due to the decrease in current efficiency in the low-concentration region, which causes the overall current efficiency to decrease.

Further, Patent Document 2 proposes a method in which Fe ²⁺ is added for treatment in order to efficiently reduce nitric acid. FIG. 6 of Patent Document 2 introduces an example of treatment using an insoluble electrode in which chloride ions coexist. On the other hand, iron is added at a high concentration of 2000 mg/L (2 g/L), and the current efficiency for nitrate reduction calculated from this example is a low value of approximately 10%. In addition, the addition of Fe ²⁺ instead of Fe ³⁺ is intended as a reducing agent for reducing nitric acid according to reaction formula 5 described later, and the reducing agent is added directly to a place where the current efficiency is low. is intended to compensate for the low current efficiency.

Further, in Patent Document 3, it is possible to reduce the nitrogen component in the nitrogen component-containing solution by electrolyzing the nitrogen component-containing solution in a state in which chloride ions, copper ions, and iron ions coexist. It is stated that it is possible. In the technique described in Patent Document 3, the role of copper ions is to suppress the generation of chlorine gas and delay the ammonia decomposition rate. Delaying the rate of decomposition of ammonia means that the hypochlorous acid produced on the anode side is consumed in the oxidation of copper and not used in the oxidation of ammonia. At this time, there is no problem if nitric acid reduction at the cathode is rate-limiting and hypochlorous acid is in excess, but if nitric acid and ammonia coexist and ammonia is present in excess In this case, the production of hypochlorous acid is rate-limiting. However, in that case, the consumption of hypochlorous acid in reactions other than ammonia oxidation has the adverse effect of causing a decrease in total efficiency.

Furthermore, in Patent Document 3, when iron ions are present alone, the effect is not as great as when copper ions are present alone, and the effect is exhibited when iron ions and copper ions coexist. There is a description to that effect. From this, it can be said that the main factor for exhibiting the action and effect is the essential presence of copper ions.

JP 2013-252508 A Japanese Patent Application Laid-Open No. 2001-252667 JP-A-2009-34633

The present invention has been proposed in view of such circumstances. It is an object of the present invention to provide a method for treating nitrogen oxides, which enables efficient electrolytic treatment even in a region.

As a result of intensive studies, the present inventors have found that by adding one or more catalysts selected from iron salts, copper salts, and nickel salts to the water to be treated containing oxide nitrogen components, the oxide nitrogen is reduced. The present inventors have found that nitrogen oxides can be decomposed and removed by efficient electrolytic treatment even in a low-concentration region, leading to the completion of the present invention.

(1) A first aspect of the present invention is a method for treating nitrogen oxides by electrolyzing an oxide nitrogen component in water to be treated containing chloride ions, wherein the water to be treated contains an iron salt, A method for treating nitrogen oxides by adding one or more catalysts selected from copper salts and nickel salts.

(2) A second invention of the present invention is the nitrogen oxide treatment method according to the second invention, wherein the catalyst is added to the water to be treated having a nitrogen oxide concentration of 1000 mg/L or less.

(3) A third aspect of the present invention is the method for treating nitrogen oxides according to the first or second aspect, wherein at least an iron salt is used as the catalyst.

(4) A fourth aspect of the present invention is, in any one of the first to third aspects, the catalyst is added so that the concentration of the catalyst in the water to be treated is 5 mg/L to 100 mg/L as a metal concentration. , a method for treating nitrogen oxides.

(5) A fifth aspect of the present invention is a method for treating nitrogen oxides according to any one of the first to fourth aspects, wherein an insoluble electrode is used as the anode.

(6) A sixth invention of the present invention is a method for treating nitrogen oxides according to any one of the first to fifth inventions, wherein an electrode having nitric acid reducing ability is used as a cathode.

(7) In a seventh aspect of the present invention, in the sixth aspect, the cathode includes, on its surface, a metal of groups 8 to 13 of the periodic table or an alloy containing the metal as a main component. , a method for treating nitrogen oxides.

(8) In an eighth aspect of the present invention, in the seventh aspect, the cathode has, on its surface, one or more alloys selected from iron-based alloys containing at least iron and nickel-based alloys containing at least nickel. It is a method for treating nitrogen oxides, including.

(9) In a ninth aspect of the present invention, in the eighth aspect, the cathode has, on its surface, an iron-chromium alloy, an iron-neodymium alloy, an iron-silicon alloy, an iron-nickel alloy, A method for treating nitrogen oxides containing one or more alloys selected from iron-tellurium alloys, nickel-chromium alloys, nickel-molybdenum alloys, and nickel-tungsten alloys.

According to the method of the present invention, by adding a specific catalyst to the water to be treated and performing electrolytic treatment, even in a low concentration nitrogen oxide region of approximately 1000 mg / L or less, for example, more efficient electricity than before Decomposition can be performed.

It is a figure which shows the structural example of the electrolytic processing apparatus used for the removal process of oxide nitrogen. It is a block diagram of the electrolytic cell in an electrolytic treatment apparatus.

Specific embodiments of the present invention will be described in detail below. In addition, the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.

≪1. Outline of the present invention>>
The method for treating nitrogen oxides according to the present invention is a method of electrochemically reducing and removing oxide nitrogen components in water to be treated containing chloride ions using an insoluble electrode. In the electrochemical reduction method, the water to be treated is introduced into an electrolytic cell having an anode and a cathode, and a voltage is applied to the electrodes to energize, so that the chloride ions are oxidized at the anode and hypochlorite An acid is generated, and the oxidized nitrogen is reduced at the cathode to generate ammonia. The generated hypochlorous acid and ammonia are reacted to remove the oxidized nitrogen as nitrogen gas.

In the method of removing oxidized nitrogen by electrolysis, the treatment method according to the present invention is characterized in that a catalyst is added to the water to be treated and electrolysis is performed. Specifically, one or more selected from iron salts, copper salts, and nickel salts is added as a catalyst. According to such a method, even when the decomposition of nitrogen oxide progresses and the concentration of nitrogen oxide components reaches a low concentration range (for example, a range of 1000 mg / L or less), nitrogen oxide can be efficiently can be reduced.

≪2. About electrolysis equipment≫
FIG. 1 is a diagram showing an outline of a configuration example of an electrolytic treatment apparatus used in a method for treating nitrogen oxides according to the present invention. As shown in FIG. 1, an electrolytic treatment apparatus 10 includes an electrolytic bath 11 for storing and electrolyzing water to be treated containing oxidized nitrogen, an adjustment bath 12 for taking in and adjusting the water to be treated, and an electrolytic bath. and a circulation pipe 13 for circulating the water to be treated between 11 and the adjustment tank 12 .

[Electrolyzer]
The electrolytic cell 11 is composed of one or more pairs of an anode 11a and a cathode 11b. FIG. 2 is a schematic diagram ((A) front view, (B) side view) of the electrolytic cell 11 . As shown in FIG. 2, the electrolytic cell 11 can be configured by a non-diaphragm electrolytic cell having no diaphragm separating the anode 11a and the cathode 11b. In the electrolytic bath 11, water to be treated 20 containing oxidized nitrogen is accommodated, and for example, a thin plate-like anode 11a and a cathode 11b are arranged so that at least a part thereof is immersed in the water to be treated 20. there is

In the electrolytic cell 11, each electrode (11a, 11b) is connected to a DC power supply 14, and by applying a DC current from the DC power supply 14 to the anode 11a and the cathode 11b, , the oxidized nitrogen in the water 20 to be treated is electrolytically removed.

Specifically, the hypochlorous acid produced at the anode 11a and the ammonia produced at the cathode 11b directly react in the electrolytic cell 11 to efficiently remove oxidized nitrogen as nitrogen gas.

(anode)
A generally used insoluble electrode can be used as the anode 11a. In order to generate hypochlorous acid from chloride ions, the anode 11a preferably employs an electrode having a high chlorine generation efficiency among insoluble electrodes. A high chlorine generation efficiency means that the oxidation reaction from chloride ions to hypochlorite ions takes precedence over other reactions at the anode. This leads to preventing the oxidized nitrogen reduced at the cathode 11b from being oxidized again.

(cathode)
The cathode 11b is an electrode used for removing nitrogen oxides in the water to be treated by electrolysis, as described above. The cathode 11b is preferably an electrode having nitric acid reducing ability. Although details will be described later, by using an electrode having nitric acid reducing ability as a cathode and adding one or more selected from iron salts, copper salts, and nickel salts as a catalyst, it is possible to reduce oxidized nitrogen in the low concentration range. high decomposition effect can be obtained.

It is preferable that the surface of the electrode having nitric acid reducing ability contains a metal of Groups 8 to 13 (Group 3B) of the periodic table or an alloy containing the metal as a main component. For example, examples of Group 8 to Group 13 metals include iron (Fe), nickel (Ni), cobalt (Co), platinum (Pt), and titanium (Ti). The electrodes may be composed only of such metals or alloys, or may be composed of conductors coated with such metals or alloys. Moreover, when the electrode is formed by coating the surface of a conductor with a metal or alloy, the conductor is not particularly limited, and examples thereof include titanium, carbon, and platinum.

In addition, as an electrode having nitric acid reduction ability, an iron-based alloy containing at least iron (Fe) as a main component (first component) or nickel (Ni) as a main component (first component) on its surface. It is preferred to use electrodes comprising nickel-based alloys. These iron-based alloys and nickel-based alloys have a relatively high nitrate reduction ability compared to other metals and their alloys.

For example, iron-based alloys include iron-chromium (Cr)-based alloys, iron-neodymium (Ne)-based alloys, iron-silicon (Si)-based alloys, iron-nickel-based alloys, iron-tellurium (Te)-based alloys, etc. is mentioned. Nickel-based alloys are preferably nickel-chromium-based alloys, nickel-molybdenum (Mo)-based alloys, nickel-tungsten (W)-based alloys, and the like. In this way, as the second component, alloys containing metal elements of Groups 4A, 5A, 6A, etc., in addition to Groups 8 and 1B of the periodic table can be mentioned. Among them, iron-chromium alloys and nickel-chromium alloys are more preferably used.

In this specification, the term "alloy" means an alloy containing at least the metal elements presented as main components, and it is only a binary alloy consisting only of these metal elements. does not mean That is, for example, in the case of an iron-chromium alloy, it is an alloy containing at least iron as the first component and chromium as the second component, and the balance may be an alloy containing one or more other metal elements. good.

Specific examples of iron-chromium alloys include iron-chromium alloys, iron-chromium-phosphorus (P)-carbon (C) alloys, iron-chromium-nickel-tantalum (Ta) alloys, and the like. Examples of iron-neodymium alloys include iron-neodymium alloys and iron-neodymium-boron (B) alloys. Examples of iron-nickel alloys include iron-nickel alloys, iron-nickel-chromium alloys, iron-nickel-boron-chromium alloys, and iron-nickel-aluminum alloys. As described above, the third and subsequent elements include alloys containing metallic elements of Groups 3B, 4B, and 5B in addition to Groups 4A, 5A, and 6A of the periodic table.

The specific composition of the alloy is not particularly limited. An example of the composition of the alloy is iron-chromium alloy, for example, in the composition formula represented by Fe80-a-bCraMobPcC20-c (at%), 10 ≤ a ≤ 40 at%, 0 ≤ b ≤ 7 at% , 5≦c≦15 at % can be used. In addition, as the nickel-chromium alloy, for example, in the composition formula represented by Ni80-d-eCrdPeB20-e (at%), an alloy that satisfies 10 ≤ d ≤ 20at% and 0 ≤ e ≤ 20at% is used. be able to.

In addition, the electrode having nitric acid reducing ability preferably contains the above-described alloy in an amorphous state on its surface. By using an electrode containing an alloy having nitrate reduction ability in an amorphous state (hereinafter also referred to as an "amorphous electrode") as the cathode 11b, corrosion resistance and wear resistance can be improved extremely effectively. That is, even if a current is applied to the cathode 11b, dissolution of the cathode 11b can be prevented, and elution of the metal component into the solution can be prevented. Moreover, even when used in an environment where electric current is applied and excessive corrosion occurs, the dissolution can be effectively prevented. As a result, it becomes an electrode that has both high reduction performance against nitrogen oxides and excellent durability (corrosion resistance and wear resistance), prevents the reduction of reduction capacity, maintains its performance, and performs extremely efficient treatment. be able to.

The method for amorphizing the alloy is not particularly limited as long as the alloy can be effectively amorphized, and known methods can be used.

[Adjustment tank]
The adjustment tank 12 periodically takes in the water to be treated 20 to be electrolyzed in the electrolytic tank 11 through the circulation pipe 13, and adjusts the pH of the water to be treated 20 and decomposes excess hypochlorous acid. I do. The water to be treated 20 adjusted in the adjustment tank 12 is circulated through the circulation pipe 13 to the electrolytic tank 11 again.

In the adjustment tank 12, a pH measuring device for measuring the pH of the water to be treated 20 taken in from the electrolytic tank 11, and hypochlorous acid for measuring the concentration of hypochlorous acid generated at the anode 11a of the electrolytic tank 11. A measuring unit 15 comprising an acid concentration measuring device or the like is provided. In addition, based on the pH and hypochlorous acid concentration of the water to be treated 20 measured by the measurement unit 15, each of the pH adjuster supply unit 16 and the hypochlorous acid scavenger supply unit 17 is supplied to the adjustment tank 12. A control unit 18 is provided to control the supply of the medicine.

In such an adjustment tank 12, when the water to be treated 20 from the electrolytic tank 11 is taken in through the circulation pipe 13, the pH and hypochlorous acid concentration of the water to be treated 20 are measured by the measurement unit 15, and the measurement A signal of the result is transmitted to the control unit 18 . Based on the received measurement results, the control unit 18 controls the pH adjuster supply unit 16 and the hypochlorous acid scavenger supply unit 17 so that the pH is within a predetermined range and the hypochlorous acid concentration is below a predetermined level. , pH adjusters and hypochlorous acid scavengers.

Regarding the pH of the water to be treated 20, when the pH becomes low, chlorine gas is generated from the hypochlorous acid produced at the anode 11a in the electrolytic cell 11, while when the pH becomes high, ammonia volatilizes. Since there are problems such as nitrate ions being easily generated when ammonia and hypochlorous acid react, the pH of the water to be treated 20 during the electrolytic treatment is generally 4.0 to 10.0, preferably is preferably maintained between 5.0 and 9.0. Therefore, in the adjustment tank 12, it is preferable to perform control by adding a pH adjuster so as to maintain the pH of the water to be treated within the above range.

[Circulation piping]
The circulation pipe 13 mainly consists of a pipe 13a for transferring the water 20 to be treated taken out from the electrolytic cell 11 to the adjusting tank 12 and a pipe 13b for returning the water 20 to be treated regulated in the adjusting tank 12 back into the electrolytic cell 11. , consists of In the circulation pipe 13 , for example, the water to be treated 20 is circulated by the circulation pump 19 .

Also, in the circulation pipe 13, a measuring unit for measuring the pH and hypochlorous acid concentration of the water 20 to be treated may be provided. Similar to the measuring unit 15 provided in the adjustment tank 12, by providing the measuring unit in the circulation pipe 13, the pH and hypochlorous acid concentration of the water 20 to be treated from the electrolytic tank 11 can be measured. The pH and hypochlorous acid concentration may be adjusted in the adjustment tank 12 into which the water 20 to be treated is introduced based on the measurement results while passing through the pipe 13 . It should be noted that such a measuring unit may be provided only in the circulation pipe 13 without being provided in the adjustment tank 12 to perform the above-described control.

≪3. Electrolytic reaction≫
Next, the electrolytic treatment reaction of the water to be treated occurring in the electrolytic cell 11 of the electrolytic treatment apparatus 10 described above will be specifically described.

When the electrodes (11a, 11b) of the electrolytic cell 11 are energized, chloride ions are oxidized to hypochlorite ions at the anode 11a as shown in Reaction Formula 1 below. On the other hand, at the cathode 11b, oxidized nitrogen is reduced to ammonia (ammonia nitrogen) as shown in Reaction Formula 2 below. In the following Reaction Formula 1 and Reaction Formula 2, the reaction equivalent per 48 electron moles is used to compare the reaction equivalents per the same amount of electricity.

[Reaction Formula 1]
24Cl ⁻ +24H ₂ O→24ClO ⁻ +48H+48e
[Reaction Formula 2]
6NO ³⁻ +48e+42H ₂ O→6NH ⁴⁺ +60OH ⁻

As shown in FIG. 1, when electrolysis is performed in a non-diaphragm electrolytic cell that does not partition the anode 11a and the cathode 11b, the hypochlorous acid generated at the anode 11a and the reduction reaction of nitrogen oxide at the cathode 11b. The ammonia produced in 1 is uniformly mixed in the electrolytic cell 11 and reacts in situ according to the following reaction formula 3 to become nitrogen gas and is removed to complete the denitrification reaction.

[Reaction Formula 3]
6NH ⁴⁺ +9ClO ⁻ →3N ₂ +9Cl ⁻ +9H ₂ O+6H ⁺

As the reduction of oxidized nitrogen proceeds, the concentration of nitrate ions in the water to be treated decreases. The reaction shown in Reaction Formula 2 above becomes less likely to proceed as the concentration of nitrate ions decreases. Therefore, when the concentration of the nitrogen oxide component in the water to be treated decreases, the reduction of the nitrogen oxide does not proceed easily even when the water is energized, resulting in a decrease in the current efficiency. This point is a phenomenon that cannot be avoided as long as the reduction of oxidized nitrogen occurs on the surface of the cathode 11b.

A decrease in current efficiency for reducing nitrogen oxide means that the supplied electricity (electric charge) is consumed by another reaction. Normally, the hydrogen generation reaction is consumed in the reduction reaction of hypochlorous acid.

Therefore, the treatment method according to the present invention is characterized by adding a catalyst composed of a specific metal salt to the water to be treated to perform electrolysis. By adding a specific catalyst in this way, part of the electricity supplied to the cathode 11b is consumed in the reduction reaction of the catalyst, and the reduced catalyst contributes to the reduction of nitric acid. . Although the detailed mechanism has not been clarified, it is presumed as follows.

(Presumed mechanism)
In the presumed mechanism below, the case where an iron salt is used as the catalyst will be described as an example, but the catalyst is not limited to the iron salt.

First, iron, which is a catalyst, is reduced by the reaction shown in Reaction Formula 4 below on the cathode 11b.
[Reaction Formula 4]
Fe ³⁺ +e ⁻ →Fe ²⁺

Next, the generated Fe ²⁺ reduces nitrate ions according to Reaction Formula 5 below.
[Reaction Formula 5]
8Fe ²⁺ +NO ³⁻ +7H ₂ O→8Fe ³⁺ +NH ⁴⁺ +10OH ⁻

The iron oxidized in Reaction Formula 5 is again reduced according to Reaction Formula 4 above, and Reaction Formulas 4 and 5 are repeated, and as a total, the form itself does not change, that is, nitric acid is used as a catalyst. to reduce. That is, the charge not used in the nitric acid reduction reaction on the cathode 11b is consumed in the iron reduction reaction, and the reduced iron contributes to the reduction of nitric acid. will improve.

If Fe ²⁺ produced in Reaction Formula 4 above reacts with hypochlorous acid produced at the anode 11a, Reaction Formula 6 below is obtained.
[Reaction Formula 6]
2Fe ²⁺ +ClO ⁻ +2H ⁺ →2Fe ³⁺ +H ₂ O+Cl ⁻

However, if such Reaction Formula 6 becomes the main reaction, iron, which is a catalyst, will not be used for the reduction of nitric acid, and the generated hypochlorous acid will not contribute to the oxidation reaction of ammonia. Both the current efficiency for reduction and the current efficiency for ammonia oxidation are lowered.

In view of the assumption of the above mechanism, the catalyst to be added to the water to be treated should be reduced at the cathode 11b and the reduced catalyst should contribute to the reduction of nitric acid. It is preferably one or more selected from salts, copper salts, and nickel salts. Among them, it is particularly preferable to use an iron salt, and a catalyst containing at least an iron salt is preferable.

In addition, although the higher the concentration of the catalyst to be added, the higher the effect can be obtained, considering the post-treatment and economic efficiency of the added catalyst, the concentration of the catalyst in the water to be treated is 5 mg / L to 100 mg / L in terms of metal concentration. It is preferable to add so that the L range is obtained.

Moreover, the cathode 11b to be used is preferably an electrode having a nitric acid reducing ability as described above. Even when an electrode having no nitric acid reducing ability is used, the effect of adding a catalyst is still there, but the effect is complementary. Therefore, an electrode having nitrate reduction ability is used as the cathode 11b, and a catalyst composed of one or more selected from iron salts, copper salts, and nickel salts is added as a so-called co-catalyst to the water to be treated to generate electricity. You can get great results by breaking it down.

Here, the treatment method according to the present invention is particularly effective when the concentration of nitrogen oxide components contained in the water to be treated is low. As described above, the examples of Patent Document 1 (Japanese Patent Application Laid-Open No. 2013-252508) show that nitrate nitrogen is effectively decomposed. It can be seen that the decomposition rate is reduced in the region below approximately 500 mg/L (Figs. 3 and 4 of Patent Document 1).

In contrast to such conventional techniques, according to the treatment method according to the present invention, one or more selected from iron salts, copper salts, and nickel salts is added as a catalyst to the water to be treated to perform electrolytic decomposition. As a result, part of the electricity supplied to the cathode 11b is consumed in the reduction reaction of the catalyst, and the reduced catalyst contributes to the reduction of nitric acid, improving the current efficiency.

Specifically, in the treatment method according to the present invention, it is preferable to treat water having a nitrogen oxide concentration of 1000 mg/L or less as the treatment object, and the water to be treated contains iron salt , a copper salt, and a nickel salt, the current efficiency can be improved and oxidized nitrogen can be efficiently decomposed and removed.

In addition, the treatment method according to the present invention is not limited to treating water having a nitrogen oxide concentration of 1000 mg/L or less, but treating water containing a higher concentration of nitrogen oxide. can also be processed as Further, for example, the water to be treated containing the nitrogen oxide component at a high concentration (for example, the concentration of the nitrogen oxide component is about 5000 mg / L) is subjected to electrolysis treatment to remove the nitrogen oxide component in the water to be treated. The treatment method of the present invention may be executed when the concentration becomes 1000 mg/L or less.

Specific examples of the present invention will be described below. In addition, the scope of the present invention is not limited to any of the following examples.

[Example 1]
In Example 1, the electrolytic treatment of nitrogen oxides was performed using the electrolytic treatment apparatus 10 (the electrolytic bath 11 is the same as in FIG. 2) shown in FIG.

In the electrolytic cell 11, a commercially available insoluble electrode for soda electrolysis is used as the anode 11a, and an electrode obtained by coating the surface of a titanium plate with an iron-chromium alloy (Fe45Cr35P13C7 (at%)) in an amorphous state (amorphous electrode) as the cathode 11b. ) was used. Water to be treated having a concentration of nitric nitrogen of 500 mg/L is contained in the electrolytic cell 11, and a direct current is passed between the anode 11a and the cathode 11b from the DC power supply 14 to convert the water to be treated containing oxidized nitrogen into Electrolytic treatment was performed on the The test was conducted by setting the areas of the current-carrying portions of the anode 11a and the cathode 11b to 0.01 m ² and setting the amount of current to be applied to 10A.

To the water to be treated having a nitrate nitrogen concentration of 500 mg/L contained in the electrolytic cell 11, 2000 mg/L of ammonium nitrogen concentration after addition of ammonium chloride as a surplus hypochlorous acid scavenger is added, and further chloride as a catalyst is added. 600 ml of a solution in which ferric iron was added so that the metal Fe concentration was 100 mg/L was put into the adjustment tank 12 in the electrolytic treatment apparatus 10, and the electrolytic treatment was performed while decomposing excess hypochlorous acid. In addition, since the pH of the water to be treated fluctuates due to the electrolysis reaction during electrolysis, caustic soda, which is a chemical for pH adjustment, was sequentially added to the adjustment tank 12 to adjust the pH of the water to be treated in order to cope with the fluctuation. .

[Example 2]
In Example 2, the same test as in Example 1 was performed, except that a titanium plate was used as the cathode 11b.

[Comparative Example 1]
In Comparative Example 1, the same test as in Example 1 was performed, except that no catalyst was added.

[Comparative Example 2]
In Comparative Example 2, the same test as in Example 2 was performed, except that no catalyst was added.

[Example 3]
In Example 3, the same test as in Example 1 was performed except that ferric chloride as a catalyst was added so that the metal Fe concentration was 50 mg/L.

[Example 4]
In Example 4, the same test as in Example 1 was performed except that ferric chloride as a catalyst was added so that the metal Fe concentration was 10 mg/L.

[Example 5]
In Example 5, the same test as in Example 1 was performed except that ferric chloride as a catalyst was added so that the metal Fe concentration was 5 mg/L.

[Example 6]
In Example 6, the same test as in Example 1 was conducted except that ferrous chloride as a catalyst was added so that the metal Fe concentration was 5 mg/L.

[Example 7]
In Example 7, the same test as in Example 1 was performed except that ferrous chloride as a catalyst was added so that the metal Fe concentration was 10 mg/L.

[Example 8]
In Example 8, the same test as in Example 1 was performed except that ferrous chloride as a catalyst was added so that the metal Fe concentration was 100 mg/L.

[Example 9]
In Example 9, the same test as in Example 1 was performed, except that cupric chloride as a catalyst was added so that the metal Cu concentration was 100 mg/L.

[Example 10]
In Example 10, the same test as in Example 1 was performed except that nickel chloride as a catalyst was added so that the metal Ni concentration was 100 mg/L.

Table 1 below shows the test results in each example and comparative example. In Table 1, regarding the electrodes used, the iron-chromium alloy amorphous electrode is simply described as "iron alloy", and the titanium plate electrode is simply described as "titanium plate". The concentration of nitrate nitrogen after electrolytic treatment was analyzed using ion chromatography.

The current efficiency shown in the results of Table 1 was calculated using the following formula. In all tests, the concentration of ammoniacal nitrogen after treatment was below the detection limit.
Nitric acid reduction current efficiency (%) =
Amount of nitric acid reduced as a result of the test x theoretical charge amount/energized charge amount x 100
Ammonia oxidation current efficiency (%) =
(Amount of nitric acid reduced in the test + initial amount of ammonia) x theoretical charge amount/energized charge amount x 100

As shown in Table 1, from the results of comparison between Example 1 and Comparative Example 1, and between Example 2 and Comparative Example 2, addition of the catalyst (Examples 1 and 2) showed that nitrate nitrogen decomposition A stimulating effect was observed. It was also confirmed that the addition of a catalyst accelerates the decomposition of nitrate nitrogen regardless of the electrode material.

However, when comparing Example 1 and Example 2, the addition of a catalyst is complementary in terms of the effect of decomposing nitrate nitrogen. It can be seen that a high decomposition effect can be obtained. This will be explained in detail below.

In Example 2, both the nitrate reduction current efficiency and the ammonia oxidation current efficiency are lower than in Example 1. When titanium, which has a lower nitrate reduction ability than the iron-chromium alloy used in Example 1, is used as an electrode, not only the nitrate reduction current efficiency at the cathode decreases, but also the ammonia oxidation current efficiency is lower than that in Example 1. to reduce. Since the same anode is used in Example 1 and Example 2, it is considered that hypochlorous acid is generated in the same manner at the anode. It can be said that this suggests that chloric acid is consumed by a reaction other than ammonia oxidation, specifically, that a reduction reaction of hypochlorous acid is occurring on the cathode side. It is considered that the reduction reaction of hypochlorous acid occurs on the cathode side because the cathode used in Example 2 has almost no nitric acid reduction ability. For this reason, in order to increase both the nitrate reduction current efficiency and the ammonia oxidation current efficiency, it is preferable not only to add a catalyst, but also to use an electrode having nitrate reduction ability as a cathode. It was found that the progress of the reaction could be suppressed more effectively.

As catalysts, an accelerating effect was recognized for each of iron salt, copper salt, and nickel salt. Among them, it was found that the iron salt has a particularly remarkable promoting effect. Furthermore, the same promotion effect was confirmed regardless of whether the iron salt was ferrous or ferric. It should be noted that this is not due to the direct action of ferrous iron as a reducing agent, but rather to the fact that the above reaction formulas 4 and 5 are repeated, and the iron salt is involved as a catalyst for nitrate reduction. is shown.

In addition, although the concentration of the catalyst to be added varies depending on the type of catalyst, the iron salt that showed the most remarkable effect is added so that the metal Fe concentration is 5 mg / L or more, so that the effect is more effective. It was confirmed that the higher the concentration, the higher the effect tended to be.

REFERENCE SIGNS LIST 10 electrolysis device 11 electrolytic bath 11a anode 11b cathode 12 adjustment bath 13 circulation pipe 14 direct current power supply device 15 measurement unit 16 pH adjuster supply unit 17 hypochlorous acid scavenger supply unit 18 control unit 19 circulation pump 20 water to be treated

Claims (6)

A nitrogen oxide treatment method for removing nitrogen oxide components in water to be treated containing chloride ions by electrolysis,
Using an insoluble electrode as the anode and an electrode having nitric acid reduction ability as the cathode,
One or more catalysts selected from iron salts, copper salts, and nickel salts are added to water to be treated whose pH is 4.0 to 10.0 , and the concentration of the catalyst is 5 mg / L to 100 mg / L as a metal concentration. A method for treating nitrogen oxides. The method for treating nitrogen oxides according to claim 1, wherein the catalyst is added to the water to be treated having a concentration of the nitrogen oxides of 1000 mg/L or less. 3. The method for treating nitrogen oxides according to claim 1, wherein at least an iron salt is used as the catalyst. 4. The method for treating oxidized nitrogen according to any one of claims 1 to 3 , wherein the surface of the cathode includes a metal of groups 8 to 13 of the periodic table or an alloy containing the metal as a main component. 5. The method for treating nitrogen oxides according to claim 4 , wherein the cathode includes, on its surface, one or more alloys selected from iron-based alloys containing at least iron and nickel-based alloys containing at least nickel. The cathode has, on its surface, an iron-chromium alloy, an iron-neodymium alloy, an iron-silicon alloy, an iron-nickel alloy, an iron-tellurium alloy, a nickel-chromium alloy, a nickel-molybdenum alloy, 6. The method for treating nitrogen oxides according to claim 5 , comprising one or more alloys selected from nickel-tungsten alloys.

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