JP4966928B2 - Water treatment method - Google Patents

Description

  The present invention relates to a water treatment method suitable for treating an organic compound dissolved in water such as industrial wastewater.

Organic compounds dissolved in water such as industrial wastewater are generally decomposed and rendered harmless by biological treatment using aerobic or anaerobic microorganisms, but when decomposition treatment using microorganisms is difficult There is also. Such persistent organic compounds can often be decomposed by accelerated oxidation treatment. In the accelerated oxidation treatment, OH radicals (hydroxy radicals: OH) are generated in the treatment apparatus, and organic compounds are decomposed into low molecules such as CO ₂ , formic acid, and aldehydes by the strong oxidizing power of the OH radicals. is there. The accelerated oxidation treatment is more effective when used with biological treatment.

Conventional accelerated oxidation treatment is generally performed by combining ozone (O ₃ ), hydrogen peroxide (H ₂ O ₂ ), and UV irradiation, and has been put to practical use for dioxin decomposition treatment. This method requires a large cost for special equipment such as an ozone generator, an exhaust gas treatment device, and a UV irradiation device. On the other hand, as a simple method that requires little initial investment and does not require special equipment, a reaction (Fenton reaction) that generates OH radicals from divalent iron ions (Fe ²⁺ ) and hydrogen peroxide as shown in the following formula is used. The Fenton method is known.
Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + OH ⁻ + OH
In this Fenton method, a divalent iron ion source and hydrogen peroxide are supplied as chemicals from the outside, and the supply cost of hydrogen peroxide, which is particularly dangerous for handling, is very high.

Furthermore, an electrolytic Fenton method is known in which the Fenton method and electrolytic treatment are combined, and both divalent iron ions and hydrogen peroxide required by the Fenton method are generated and supplied in the same apparatus as the accelerated oxidation treatment. Yes. There are the following methods for the main production and supply of divalent iron ions in the electrolytic Fenton method.
(1) Depending on the cathode (cathode)
Fe ³⁺ + e ⁻ → Fe ²⁺
A method of supplying divalent iron ions by electrolytic reduction of trivalent iron ions generated by the Fenton reaction.
(2) Using an iron electrode for the anode (anode),
Fe → Fe ²⁺ + 2e ⁻
A method to generate divalent iron ions by electrolysis.
In addition, the main production and supply method of hydrogen peroxide is by the cathode,
O ₂ + 2H ⁺ 2e ⁻ → H ₂ O ₂
There is a method of supplying hydrogen peroxide by electrolytic reduction of oxygen. In such an electrolytic Fenton method, it is not necessary to input a divalent iron ion source and / or hydrogen peroxide from the outside as a medicine except for the initial charging. The supply method can be expected to reduce the supply cost of hydrogen peroxide.

On the other hand, in order to generate OH radicals, Non-Patent Document 1 introduces a divalent iron ion source and hypochlorous acid (HOCl), and a reaction similar to the Fenton reaction represented by the following formula (Fenton): Type reaction) has been reported.
Fe ²⁺ + HOCl → Fe ³⁺ + Cl ⁻ + OH
According to Non-Patent Document 1, this Fenton type reaction has a feature that the reaction rate constant is about three orders of magnitude larger than that of the Fenton reaction.

L. P. Candias, two others, “Formation of hydroxyl radicals on reaction of hypochlorous acid with ferrocyanide, a model iron (2) complex”, Free Rad. Res. , (USA), Harwood Academic Publishers GmbH, 1994, Vol. 20, No. 4, p. 241-249

  However, when used for water treatment of industrial wastewater, etc., the above Fenton method, as well as the electrolytic Fenton method of electrolytically reducing oxygen and supplying hydrogen peroxide, promote pure oxygen gas from gas cylinders, etc. It is necessary to supply into the oxidation treatment apparatus, and it cannot be said that reduction of the supply cost of the chemical can be sufficiently achieved.

Further, the technology for causing the Fenton-type reaction using hypochlorous acid instead of hydrogen peroxide as in Non-Patent Document 1 cannot be said to be practically used as it is as described below. The inventor of the present application experimented as follows as to whether or not a technique for introducing a divalent iron ion source and hypochlorous acid to cause a Fenton-type reaction as in Non-Patent Document 1 can be used. 1,4-Dioxane was used as the organic compound to be decomposed. A solution A containing 1.25 mM of 1,4-dioxane and 4 mM of hypochlorous acid and a solution B containing 4 mM of iron sulfate (FeSO ₄ ) as a divalent iron ion source were prepared, and they were mixed in an equivalent amount to obtain a Fenton-type reaction. And the amount of 1,4-dioxane decomposed was measured. The initial concentration of 1,4-dioxane is 0.625 mM. On the other hand, for comparison, a solution A ′ containing 1.25 mM of 1,4-dioxane and 4 mM of hydrogen peroxide and a solution B ′ containing 4 mM of iron sulfate were prepared, and each was mixed in an equivalent amount to perform the Fenton reaction. The amount of 1,4-dioxane decomposed was measured. The initial concentration of 1,4-dioxane is 0.625 mM. The pH was controlled to be around 2 by adding an appropriate amount of sulfuric acid.

  Stoichiometrically, it is expected that the amount of OH radicals produced is the same when the solution A and the solution B are mixed and when the solution A ′ and the solution B ′ are mixed. The amount of decomposition is expected to be equal. The actual experimental results show that the decomposition amount of 1,4-dioxane when the solution A and the solution B are mixed is 0.0967 mM, and the decomposition amount of 1,4-dioxane when the solution A ′ and the solution B ′ are mixed is 0.518 mM. That is, when the solution A and the solution B were mixed, the decomposition amount of 1,4-dioxane was only about one-fifth compared with the case where the solution A ′ and the solution B ′ were mixed. This acts as a radical scavenger that consumes the OH radicals in which hypochlorous acid remaining in a high concentration state in the solution A due to the large reaction rate constant of OH radicals and hypochlorous acid, This is probably because the amount of OH radicals used for the decomposition of 1,4-dioxane was reduced.

  Therefore, the present inventor has found that industrial wastewater or the like originally contains a large amount of chloride ions, and that the chloride ion source is an inexpensive and low-risk method (typically sodium chloride ( Paying attention to the fact that it can be supplied (in the form of NaCl), hypochlorous acid is gradually generated using chloride ions, and then OH radicals are generated to efficiently decompose the organic compound. We researched practical methods using mold reaction.

  The present invention has been made in view of such reasons, and its purpose is to produce low-cost water that can efficiently decompose organic compounds by generating OH radicals from divalent iron ions and hypochlorous acid. It is to provide a processing method.

In the water treatment method according to claim 1, iron ions and chloride ions are present in the water to be treated injected into the water tank from the added iron ion source and the added chloride ion source, and the pH is reduced to 3 or less. While adjusting so that a predetermined current set by applying a voltage between the anode and the cathode immersed in the water to be treated is allowed to react with the chloride ions and water at the anode, Produces chlorous acid, reduces trivalent iron ions at the cathode to produce divalent iron ions, and reacts with these hypochlorous acid and divalent iron ions to produce water to be treated. It includes an accelerated oxidation step in which dissolved organic compounds are oxidized and decomposed .

The water treatment method according to claim 2 is characterized in that, in the water treatment method according to claim 1 , the current density between the anode and the cathode is set to 8.5 mA / cm ² or less in the accelerated oxidation step. And

The water treatment method according to claim 3 is the water treatment method according to claim 1 or 2 , wherein iron hydroxide is precipitated by adding a neutralizing agent to the water tank to neutralize the water to be treated. The method further comprises a neutralization step and a water discharge step of discharging the water to be treated while leaving the precipitated iron hydroxide.

The water treatment method according to a fourth aspect is the water treatment method according to the third aspect , wherein a period for applying a voltage is provided between the anode and the cathode in the neutralization step.

  According to the water treatment method of the present invention, hypochlorous acid is gradually generated by reaction of chloride ions and water at the anode, and trivalent iron ions are reduced at the cathode to produce divalent iron ions. These hypochlorous acid and divalent iron ions react to generate OH radicals, and the organic compounds can be efficiently decomposed by the OH radicals. As a result, low-cost water treatment becomes possible.

The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of a water treatment apparatus 1 that realizes a water treatment method according to an embodiment of the present invention. This water treatment apparatus 1 has a water tank 2, and an anode (anode) 3X and a cathode (cathode) 3Y are installed inside the water tank 2, and a voltage is applied between the anode 3X and the cathode 3Y outside the water tank 2. A voltage applying means 4 for applying is provided. The anode 3X is, for example, a RuO ₂ / Ti electrode, and the cathode 3Y is, for example, a stainless steel electrode. Moreover, FIG. 2 is explanatory drawing for demonstrating the flow between each process of this water treatment method. In this water treatment method, the water to be treated S such as industrial wastewater injected into the water tank 2 is an accelerated oxidation step shown in FIG. 2 (a), a neutralization step shown in FIG. 2 (b), and FIG. It is processed through a water discharge process indicated by).

In the accelerated oxidation step, an iron ion source (for example, iron chloride (FeCl ₃ )) is added to the water to be treated S in the water tank 2, and a chloride ion source (for example, sodium chloride (NaCl)) as necessary. As a result, iron ions and chloride ions are present in the water to be treated S. If the treated water S originally contains a large amount of chloride ions, a chloride ion source added from the outside is not necessary. Then, while adjusting the pH to be acidic, the current value is set to a predetermined value, and a voltage is applied between the anode 3X and the cathode 3Y. As will be described later, iron hydroxide obtained in the neutralization step can be reused as an iron ion source. Therefore, when performing the accelerated oxidation step by injecting new treated water S after the water discharge step, the iron ion source External addition will be performed when insufficient.

By applying a voltage between the anode 3X and the cathode 3Y, hypochlorous acid (HOCl) is reacted with chloride ion (Cl ⁻ ) and water (H ₂ O) at the anode 3X as shown in the following formula. Accompanying this, hydrochloric acid (HCl) is generated, and electrons remain in the anode 3X.
2Cl ⁻ + H ₂ O → HOCl + HCl + 2e ⁻

In the cathode 3Y, as shown in the following formula, trivalent iron ions (Fe ³⁺ ) in the water to be treated S including those generated by the Fenton-type reaction described later receive electrons from the cathode 3Y and A reduction reaction to produce valent iron ions (Fe ²⁺ ) occurs.
Fe ³⁺ + e ⁻ → Fe ²⁺

Hypochlorous acid generated at the anode 3X reacts with divalent iron ions in the treated water S, that is, divalent iron ions generated at the cathode 3Y, as shown by the following formula (Fenton type reaction). OH radicals and accompanying chloride ions and trivalent iron ions are generated.
Fe ²⁺ + HOCl → Fe ³⁺ + Cl ⁻ + OH
The organic compound dissolved in the water to be treated S is accelerated and oxidized by the OH radicals and decomposed. Here, hypochlorous acid is gradually generated at the anode 3X, OH radicals are generated from the hypochlorous acid, and most of the OH radicals are subjected to decomposition of the organic compound. Therefore, the organic compound can be efficiently decomposed.

In the accelerated oxidation step, the pH is preferably 3 or less, as will be described in the experimental results described later. Further, the current density of the current flowing between the anode 3X and the cathode 3Y is preferably set to 8.5 mA / cm ² or less in terms of current efficiency, as will be described in the experimental results described later. Moreover, it is preferable to set to 1 mA / cm < ² > or more from the point of the daily processing amount.

  In the neutralization step, a neutralizing agent (for example, sodium hydroxide (NaOH)) is added to the water to be treated S of the water tank 2 in which the accelerated oxidation step has been completed, and neutralized so that the pH becomes 4 or more. By doing so, a precipitate D of iron hydroxide is produced. In addition, a period for applying a voltage between the anode 3X and the cathode 3Y is provided, and a current is continuously passed between the anode 3X and the cathode 3Y to promote the generation of trivalent iron ions, thereby causing precipitation of iron hydroxide. The generation of D can be accelerated. In the water discharge step, the water to be treated is discharged leaving the precipitate D. In addition, although it is possible to perform a post-treatment on the discharged water to be treated, it is preferable to set the pH to 8 or less when the post-treatment is not performed. Thereafter, in the state where the precipitate D is present, when new water to be treated S is injected and the pH is adjusted to be acidic, iron ions are generated from iron hydroxide. In this way, iron hydroxide is reused as iron ions, and the new treated water S is similarly subjected to the accelerated oxidation step, neutralization step, and water discharge step.

  Thus, since this water treatment method uses the simple water treatment apparatus 1, the initial cost can be reduced. In addition, the iron ion source used in this water treatment method can reuse iron hydroxide obtained in the neutralization step, and the chloride ion source added from the outside has a large amount of chloride ions in the treated water S originally. If it is contained, it is not necessary. Even when a chloride ion source is added from the outside, a chloride ion source such as sodium chloride is inexpensive, and an iron ion source or a chloride ion source is hydrogen peroxide. The operating cost can be reduced because it is not a chemical with high handling risk.

  In addition, the specific form of the water treatment apparatus 1 depends on the type, amount or location of the water to be treated S. For example, when the wastewater of the factory in the vicinity of the coast is the treated water S, it is possible to add seawater as the chloride ion source to the treated water S.

  Next, an experiment conducted by the present inventor will be specifically described below.

FIG. 3 is a schematic diagram showing the configuration of the experimental apparatus for this experiment. The anode 3X and the cathode 3Y installed inside the water tank 2 were plate-like electrodes having an effective electrode area of 72.3 cm ² . The anode 3X is a RuO ₂ / Ti electrode, and the cathode 3Y is a stainless steel (SUS304) electrode. The voltage applying means 4 sets a current value and applies a voltage (about 4 V) between the anode 3X and the cathode 3Y according to the value. For stirring the treated water S, a magnetic stirrer 5 is provided. The magnetic stirrer 5 includes a stirrer 5A that is placed on the inner bottom of the water tank 2 and rotates to stir the water to be treated S, and a magnetic stirrer body 5B that drives the stirrer 5A. In order to adjust the pH, a sulfuric acid container 6A containing sulfuric acid for adjusting pH (H ₂ SO ₄ ), a pump 6B for sending sulfuric acid from the sulfuric acid container 6A to the water tank 2, A pH sensor 6C that measures the pH by being immersed in the water to be treated S, and a pH controller 6D that controls the operation / non-operation of the pump 6B based on the measured value are provided.

1,4-Dioxane was used as the organic compound to be decomposed. 1 L of treated water S in which 1,4-dioxane is dissolved is injected into the water tank 2, sodium chloride (NaCl) and iron chloride (FeCl ₃ ) are added, and 1,4-dioxane is 20 mM in the initial state. , Cl ⁻ was 281 mM, and trivalent iron ions were adjusted to a predetermined concentration according to each experiment described later. In the experiment of the accelerated oxidation process, while the water to be treated S is stirred and adjusted to have a pH of 2, the current value is set to a predetermined value according to each experiment, and a voltage is applied between the anode 3X and the cathode 3Y. And the concentration of 1,4-dioxane was measured.

First, an experiment of the accelerated oxidation process when the current density of the current flowing between the anode 3X and the cathode 3Y is changed as a parameter will be described. FIG. 4 shows the temporal transition of the concentration of 1,4-dioxane at that time. The concentration of trivalent iron ions in the initial state was 2 mM. The current was changed to 0.1A, 0.2A, 0.5A, 1.0A, 1.5A, 2.0A. Therefore, the current density is 1.38 mA / cm ² , 2.77 mA / cm ² , 6.92 mA / cm ² , 13.8 mA / cm ² , 20.8 mA / cm ² , 27.7 mA / cm ² The data a, b, c, d, e, and f in the middle correspond to each other).

From FIG. 4, the concentration of 1,4-dioxane decreases almost linearly at any current density, that is, in the zeroth order reaction. For example, when the current density is 6.92 mA / cm ² , the concentration decreases to 5 after 60 minutes. .36 mM (27%) is decreasing. From the decrease in the zero-order reaction, it can be seen that hypochlorous acid is continuously generated by electrolysis at the anode 3X and divalent iron ions are continuously generated at the cathode 3Y.

In data with a current density up to 6.92 mA / cm ² , the decrease rate of 1,4-dioxane increases as the current density increases. However, the decrease rate of 1,4-dioxane is smaller at a current density of 13.8 mA / cm ² than when it is 6.92 mA / cm ² . Furthermore, the decrease rate of 1,4-dioxane tends to decrease as the current density increases. Here, when the state of the cathode 3Y when the current density was 13.8 mA / cm ² or more was observed, reddish brown deposits were observed on the surface thereof. From this, it is considered that when the current density is 13.8 mA / cm ² or more, the pH in the vicinity of the cathode 3Y increases with the electrolysis of water, and iron hydroxide is adhered to the surface of the cathode 3Y. As a result, it is considered that the production of divalent iron ions at the cathode 3Y was suppressed, and the divalent iron ions necessary for the Fenton reaction were not sufficiently provided.

FIG. 5 shows the current efficiency with respect to the current density obtained from the data of FIG. The horizontal axis is a LOG scale. The current efficiency CE was determined by the following formula.
CE = 100 × ((C ₀ −C) × V) ÷ (I × T ÷ 2F)
C ₀ and C are the initial concentration of 1,4-dioxane and the concentration at the time of measurement, respectively. V is the volume of treated water S (ie, 1 L), I is the current value, T is the energization time, and F is the Faraday constant (ie, 96485 coulomb / mol). (C ₀ -C) × V is an actual decrease amount of 1,4-dioxane. I × T ÷ 2F is a theoretical value of the amount of OH radicals generated from the flowing current, that is, the amount of decrease in 1,4-dioxane. Dividing by 2 is because 2 mol of electrons are required to produce 1 mol of hypochlorous acid.

From FIG. 5, when the current density is 1.38 mA / cm ² and 2.77 mA / cm ² , the current efficiency is close to 100%. Therefore, OH radicals are generated and organic compounds are efficiently decomposed. I understand that. Also, current efficiency tends to decrease with increasing current density. The current efficiency decreases rapidly when the current density is between 6.92 mA / cm ² and 13.8 mA / cm ² . FIG. 5 shows that a current efficiency of 50% or more can be obtained when the current density is 8.5 mA / cm ² or less. Accordingly, in terms of current efficiency, the current density is preferably set to 8.5 mA / cm ² or less.

On the other hand, if the current density is low, the current efficiency is high, but naturally, the decrease rate of 1,4-dioxane is also small as shown in FIG. If the current density is 1 mA / cm ² or more, it is considered possible to complete the processing of all 1,4-dioxane in an appropriate operating time of one day, so in terms of throughput, the current density is It is preferable to set it to 1 mA / cm ² or more.

Next, an experiment of the accelerated oxidation process when the trivalent iron ion concentration in the initial state is changed as a parameter will be described. FIG. 6 shows the time transition of the concentration of 1,4-dioxane at that time. FIG. 6A shows a current density set to 1.38 mA / cm ² , and FIG. 6B shows a current density set to 6.92 mA / cm ² . The trivalent iron ion concentration was changed to 0.5 mM, 1.0 mM, 2.0 mM, 4.0 mM (data g, h, i, and j in the figure correspond to each).

  In FIG. 6 (a), the iron ion concentration dependency is not recognized for the decrease rate of 1,4-dioxane, and in FIG. 6 (b), the decrease rate of 1,4-dioxane increases as the iron ion concentration increases. Was recognized. Under the condition where the current density is low, the electrolytic supply of hypochlorous acid is rate-limiting, whereas under the condition where the current density is high, the diffusion of iron ions in the vicinity of the anode 3X is considered to be rate-limiting.

In FIG. 6B, when the trivalent iron ion concentration is 1 mM or more, the decrease rate of 1,4-dioxane is not so great even if the trivalent iron ion concentration is increased. Therefore, considered that until the current density 6.92mA / cm ² above the current density of 8.5 mA / cm ² current density slightly greater than can relieve rate-limiting due to diffusion of iron ions if the iron ion concentration of more than 1mM It is done.

Next, an experiment of the accelerated oxidation process when the pH is adjusted to 4 will be described. FIG. 7 shows the temporal transition of the concentration of 1,4-dioxane at that time. The current density is 6.92 mA / cm ² . As shown in FIG. 7, 1,4-dioxane was hardly reduced. When the pH was adjusted to 3, it was almost the same as when the pH was 2.

Next, the experiment of the neutralization process after the accelerated oxidation process and the water discharge process will be described. Sodium hydroxide (NaOH) is added as a neutralizing agent to the water to be treated S in the water tank 2 in which the accelerated oxidation process has been completed under conditions of an initial trivalent iron ion concentration of 2 mM and a current density of 6.92 mA / cm ^2. The solution was neutralized so that the pH was 4 or more (neutralization step). Then, a voltage was applied between the anode 3X and the cathode 3Y continuously for 10 minutes to generate a precipitate D of iron hydroxide. The deposit D was left and the treated water S was discharged as treated water (water discharge step). Here, the recovery rate of iron in the form of iron hydroxide precipitate D was found to be about 95%. Furthermore, in the state where the precipitate D is present, a new treatment water S is injected, sodium chloride is added, and an experiment of the accelerated oxidation process is performed while adjusting the pH to be 2. It was confirmed that Thereby, it turns out that the iron hydroxide obtained by the neutralization process after an acceleration | stimulation oxidation process is recyclable as an iron ion.

  As mentioned above, although the water treatment method which concerns on embodiment of this invention was demonstrated, this invention is not restricted to what was described in the above-mentioned embodiment, Various in the range of the matter described in the claim Design changes are possible. For example, it is possible to perform only the accelerated oxidation process and perform a process similar to or different from the neutralization process and the water discharge process in another apparatus. Of course, the organic compound to be decomposed is not limited to 1,4-dioxane used in the experiment.

It is a schematic diagram of the water treatment apparatus which implement | achieves the water treatment method which concerns on embodiment of this invention. It is explanatory drawing for demonstrating the flow between each process of the water treatment method same as the above. It is a schematic diagram which shows the structure of the experiment apparatus of experiment of a water treatment method same as the above. It is a characteristic view which shows the time transition of the density | concentration of the organic compound of one experiment same as the above. It is a characteristic view which shows the current efficiency with respect to the current density of one experiment same as the above. It is a characteristic view which shows the time transition of the density | concentration of the organic compound of another experiment same as the above. It is a characteristic view which shows the time transition of the density | concentration of the organic compound of another experiment same as the above.

DESCRIPTION OF SYMBOLS 1 Water treatment apparatus 2 Water tank 3X Anode 3Y Cathode 4 Voltage application means S Water to be treated

Claims (4)

Allowed presence of chloride ions and iron ions from added with added iron ion source source of chloride ions in the water to be treated is injected into the water tank, while adjusting so that the 3 below pH, the water to be treated By passing a current of a predetermined value set by applying a voltage between the anode and the cathode immersed in
Reaction of chloride ion and water at the anode to produce hypochlorous acid,
Reducing trivalent iron ions at the cathode to produce divalent iron ions,
Water treatment characterized by including an accelerated oxidation step in which organic compounds dissolved in water to be treated are accelerated and decomposed by OH radicals generated by reacting hypochlorous acid with divalent iron ions. Method. The water treatment method according to claim 1 ,
In the accelerated oxidation step, the water treatment method is characterized in that the current density between the anode and the cathode is set to 8.5 mA / cm ² or less. The water treatment method according to claim 1 or 2 ,
A neutralization step of precipitating iron hydroxide by neutralizing the water to be treated by adding a neutralizing agent to the water tank;
A water discharge process for discharging the treated water leaving the precipitated iron hydroxide,
A water treatment method, further comprising: The water treatment method according to claim 3 ,
In the neutralization step, a period for applying a voltage between the anode and the cathode is provided.

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