JP2005185901A - Method and apparatus for removing heavy metals from complex heavy metal contaminated soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treatment system for efficiently removing contaminated heavy metals from washing waste water of composite heavy metal contaminated soil containing hexavalent chromium, selenium and / or arsenic.
SOLUTION: As for the composite heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, as a first step, the contaminated soil is washed into a slurry, and this slurry has a metal having a lower standard electrode potential than hexavalent chromium. In order to reduce hexavalent chromium and promote elution of selenium and / or arsenic, as a second step, a water-soluble sulfate and a water-soluble barium salt are added to the washing waste water after the first step. To form a starch containing selenium and / or arsenic together with iron hydroxide and barium, and then, as a third step, add a water-soluble sulfate to the treated waste water after the second step. A method and apparatus for removing heavy metals from complex heavy metal contaminated soil, characterized by starching and separating residual barium and iron.
[Selection] Figure 1

Description

  The present invention relates to a method and apparatus for removing these heavy metals from soil contaminated by complex heavy metals containing selenium and / or arsenic with hexavalent chromium. The heavy metal removal system of the present invention effectively pollinates and separates heavy metals such as chromium, selenium, and arsenic that are contained in water or chemicals that have washed contaminated soil, and reduces the concentration of these heavy metals contained in wastewater to the environment. It is useful as a means to make it below the standard.

  In the redevelopment of factory sites, soil contamination due to toxic substances such as hexavalent chromium, selenium, and arsenic contained in soil has become a problem. In order to remove these contaminated heavy metals from contaminated soil, it is common to wash the contaminated soil with water or chemicals to elute the contained heavy metals. It is necessary to remove the contaminated heavy metals contained in the washing waste water to reduce it to an environmental regulation value or less because it cannot be discharged outside without being treated.

Among such contaminated heavy metals in waste water, for example, the following treatment method is known for selenium. Usually, selenium is present in the state of selenic acid (H ₂ SeO ₄ ) or selenious acid (H ₂ SeO ₃ ) in water. To such selenium-containing wastewater, ferric chloride is added to precipitate monovalent selenium-containing ions (HSeO ₃ ⁻ , HSeO ₄ ⁻ ), and barium chloride and sulfuric acid are further added to add divalent selenium-containing ions. (SeO ₃ ²⁻ , SeO ₄ ²⁻ ) is precipitated and separated from the waste water by solid-liquid separation with barium sulfate starch (Patent Document 1).

Regarding the treatment of hexavalent chromium, hydrogen sulfide or aqueous sulfite solution is introduced into the waste water to reduce the hexavalent chromium to a trivalent chromium compound (Cr ₂ O ₃ · nH ₂ O) and remove the precipitate. , Using a treatment agent obtained by mixing metal zinc powder or metal iron powder with alumina-silica powder (for example, quartz porphyry powder) containing silicic acid as a main component and further reacting by adding sulfuric acid, A treatment method is known in which chromium is precipitated and separated by adding this treatment agent to hexavalent chromium-containing wastewater (Patent Document 2).
Japanese Patent Laid-Open No. 11-221576 Japanese Patent Application Laid-Open No. 07-108280

  The conventional treatment method described above is a method for separating and removing selenium, hexavalent chromium, etc. individually. When hexavalent chromium, selenium, etc. are contained in combination, each treatment method must be carried out successively. In addition, the processing steps are complicated. In addition, the treatment process for one contaminated metal may interfere with the treatment for the other contaminated metal. For example, in the treatment method of Patent Document 1, when hexavalent chromium is contained in the wastewater, the hexavalent chromium is not insolubilized, so that the concentration of hexavalent chromium in the final treated wastewater is still high. . Also, selenium is not sufficient as a treatment method because if it has a high selenite concentration, it may be dissolved again if the pH is lower than 5 even after precipitation as a barium salt.

  The present invention solves the above-mentioned problem in such a conventional treatment method, and with regard to contaminated soil containing a combination of contaminated heavy metals, the composite heavy metals contained in the washed waste water of the contaminated soil are efficiently processed by a simple treatment process. It is an object of the present invention to provide a treatment method capable of being precipitated and separated, and capable of reducing the concentration of heavy metals in the final treated wastewater to be lower than the environmental regulation value.

According to the present invention, the following method and apparatus for removing contaminated heavy metals are provided.
(1) For complex heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, as a first step, the contaminated soil is washed into a slurry, and this slurry has a metal having a lower standard electrode potential than hexavalent chromium. Water-soluble sulfate is added to reduce hexavalent chromium and promote elution of selenium and / or arsenic. As a second step, water-soluble sulfate and water-soluble barium salt are added to the washing waste water after the first step. To form and separate a starch containing selenium and / or arsenic together with iron hydroxide and barium, and as a third step, add water-soluble sulfate to the treated wastewater after the second step A method for removing heavy metals from complex heavy metal contaminated soil, characterized in that barium and iron are starched and separated.
(2) In the first step, ferrous sulfate or stannous sulfate is used as the sulfate, and the pH of the contaminated soil slurry is adjusted to 4 to 9 to reduce chromium and elute selenium and / or arsenic. The method for removing heavy metals described in (1).
(3) In the second step, a water-soluble sulfate is added so that sulfate ions having a concentration of 50 to 1000 times the selenate concentration and / or arsenate concentration are present, and then the water-soluble barium salt is added to a sulfate ion concentration of 1 0.5 to 10 times, and ferric chloride is added to a concentration of 100 to 10000 mg / L, and the pH is adjusted to 5.5 to 10 to adjust iron hydroxide and barium. A method for removing heavy metals according to (1) or (2) above, wherein a starch containing selenium and / or arsenic is produced and separated.
(4) In the third step, sulfate ion of 1.5 to 2 times the concentration of dissolved barium is added, and ferric chloride is further added, and the pH is adjusted to 5.5 to 10, so that iron hydroxide and residual The method for removing heavy metals as described in (1), (2) or (3) above, wherein a barium-containing starch is produced and separated.
(5) First thickener for washing complex heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, means for adding a water-soluble sulfate as a reducing agent to the first thickener, washing waste water extracted from the first thickener A second thickener for storing the precipitating agent and a flocculant in the second thickener, a third thickener for storing the treated waste water extracted from the second thickener, a means for adding the precipitating agent and the flocculant to the third thickener, An apparatus for removing heavy metals from a composite heavy metal contaminated soil, characterized by having an exhaust line for extracting starch from each thickener.
(6) The heavy metal removing device according to (5), wherein an ultrasonic transducer is provided at least in the first thickener.

[Detailed Description of the Invention]
In the treatment method of the present invention, as for the composite heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, as a first step, the contaminated soil is washed into a slurry, and a standard electrode potential is added to the slurry from hexavalent chromium. The addition of water-soluble sulfates with low metal content promotes the reduction of hexavalent chromium and promotes elution of selenium and / or arsenic. A barium salt is added to produce a selenium and / or arsenic-containing starch together with iron hydroxide and barium, and as a third step, water-soluble sulfate is added to the treated wastewater after the second step. It is a method for removing heavy metals from composite heavy metal contaminated soil, characterized by adding and starching the remaining barium and iron to separate them.

  In the treatment method of the present invention, the contaminated soil is first washed into a slurry as the first step. Specifically, the contaminated soil is put into the first thickener, and 2.5 to 20 times as much water as the soil dry weight ratio is added to form a slurry. A metal water-soluble sulfate having a standard electrode potential lower than that of hexavalent chromium is added to the slurry through a pipe 11. As the water-soluble sulfate, ferrous sulfate or stannous sulfate is preferably used. The pH of this soil slurry is preferably in the range of 4-9. The slurry is preferably stirred for 0.5 hour or longer. In this case, it is more preferable that an ultrasonic vibrator is attached to the bottom surface or wall surface of the first thickener to apply ultrasonic vibration to the contaminated soil inside the thickener.

When ferrous sulfate or the like is added to the contaminated soil slurry, hexavalent chromium is reduced to trivalent chromium by divalent iron ions, for example, as in the following formula, and rendered harmless. In addition, selenate ions, selenite ions, or arsenate ions and arsenite ions adsorbed on the soil particles are separated from the soil particles by the action of sulfate ions, and elution into the slurry is promoted. Is done.
CrO ₄ ^2- + 3Fe ²⁺ + 8H ⁺ → Cr ³⁺ + 3Fe ³⁺ + 4H ₂ O
[Soil particles] ・ (SeO ₄ ) n ＋ n ・ SO ₄ ^2- → [Soil particles] ・ (SO ₄ ) n + n ・ SeO ₄ ^2-
[Soil particles] ・ (SeO ₃ ) n ＋ n ・ SO ₄ ^2- → [Soil particles] ・ (SO ₄ ) n + n ・ SeO ₃ ^2-
[Soil particles] ・ (AsO ₄ ) n ＋ (3/2) n ・ SO ₄ ^2- → [Soil particles] ・ (SO ₄ ) (3/2) n + n ・ AsO ₄ ^3-
[Soil particles] ・ (AsO ₃ ) n ＋ (3/2) n ・ SO ₄ ^2- → [Soil particles] ・ (SO ₄ ) (3/2) n + n ・ AsO ₃ ^3-

  The soil is allowed to settle after the reduction reaction and the elution reaction, and the soil is settled to the tank bottom, and the settled soil is extracted from the tank bottom. The removed washed soil is neutralized or insolubilized as necessary, and used for backfilling or reclamation. On the other hand, since the contaminated heavy metal is contained in the washing wastewater remaining in the first thickener by such solid-liquid separation, it is sent to the second step to precipitate and separate the contaminated heavy metal.

  In the second step, the cleaning wastewater sent from the first thickener is supplied to the second thickener, water-soluble sulfate and water-soluble barium salt are added to this cleaning wastewater, and selenium and / or iron hydroxide and barium are added. A starch containing arsenic is produced and separated. Specifically, for example, a water-soluble sulfate is added as necessary so that sulfate ions 50 to 1000 times the selenate concentration and / or arsenate concentration are present, and the mixture is stirred for 0.5 hour or more. As this water-soluble sulfate, sodium sulfate, sodium hydrogen sulfate, potassium sulfate, potassium hydrogen sulfate, ammonium sulfate and the like are preferably used.

  Next, a water-soluble barium salt (barium chloride, barium nitrate, etc.) is added so as to have a sulfate ion concentration of 1.5 to 10 times, and the mixture is stirred for 0.5 hour or more. Thus, by adding an excess amount of barium, the precipitation reaction between selenium and / or arsenic oxoacids and barium is promoted. If an excessive amount of sulfate radicals are present, the production of barium sulfate has priority, and the precipitation reaction between selenium and / or arsenic oxoacids and barium does not proceed. Further, ferric chloride is added to a concentration of 100 to 10000 mg / L and stirred for 0.5 hour or more. During these steps, the pH of the washing waste water is adjusted to 5.5-10.

By the above treatment in the second step, selenate ion, selenite ion, arsenate ion and arsenite ion in the waste water react with barium ion and iron ion, respectively, and precipitate together with iron hydroxide. An example of this reaction is shown in the following formula.
SO ₄ ^2- + Ba ²⁺ → BaSO ₄
SeO ₄ ^2- ＋ Ba ²⁺ → BaSeO ₄
SeO ₃ ^2- ＋ Ba ²⁺ → BaSeO ₃
2AsO ₄ ^3- ＋ 3Ba ²⁺ → Ba ₃ (AsO ₄ ) ₂
2AsO ₃ ^3- ＋ 3Ba ²⁺ → Ba ₃ (AsO ₃ ) ₂
^{Fe 3+ + 3OH - + n ·} BaSO 4 → Fe (OH) 3 · (BaSO 4) n ↓
^{Fe 3+ + 3OH - + n ·} BaSeO 4 → Fe (OH) 3 · (BaSeO 4) n ↓
^{Fe 3+ + 3OH - + n ·} BaSeO 3 → Fe (OH) 3 · (BaSeO 3) n ↓
^{Fe 3+ + 3OH - + n ·} BaHAsO 4 → Fe (OH) 3 · [Ba 3 (AsO 4) 2] n ↓
^{Fe 3+ + 3OH - + n ·} BaHAsO 3 → Fe (OH) 3 · [Ba 3 (AsO 3) 2] n ↓

  The reaction is allowed to stand after the reaction to allow the starch to settle on the tank bottom of the second thickener, and the precipitated starch is extracted from the tank bottom. On the other hand, since excess barium ions are added to the second thickener as described above, the treated wastewater remaining in the second thickener is sent to the third step for treatment.

In the third step, the treated wastewater sent from the second thickener is supplied to the third thickener, and the remaining barium ions and iron ions are converted into starch by the addition of a water-soluble sulfate to the washing wastewater and separated. Specifically, for example, sulfate ions of 1.5 to 2 times the dissolved barium ion concentration are added and stirred. As the sulfate ion source, sodium sulfate or the like can be used. Further, ferric chloride is added, the pH is adjusted to 5.5 to 10, and the mixture is stirred for 0.5 hour or more to produce a starch containing iron hydroxide and residual barium. An example of this reaction is shown below.
SO ₄ ^2- + Ba ²⁺ → BaSO ₄
^{Fe 3+ + 3OH - + n ·} BaSO 4 → Fe (OH) 3 · (BaSO 4) n

  After the reaction, the mixture is allowed to stand to allow the starch to settle on the bottom of the third thickener, and is extracted outside. The treated wastewater remaining in the third thickener is drained after adjusting the pH. Chromium is detoxified by the above treatment steps, and selenium and arsenic are precipitated and removed, so the concentration of harmful selenium and arsenic remaining in the final treatment wastewater is extremely low, and it is added as a precipitating agent. Since barium is also finally precipitated and removed, the concentration of these heavy metals remaining in the waste water can be reduced below the environmental standard.

  The method and apparatus for removing heavy metals of the present invention effectively starches and separates heavy metals such as chromium, selenium, and arsenic, which are the source of contamination, without being affected by sulfate ions contained in water or chemicals that have washed contaminated soil. The concentration of these heavy metals contained in the waste water can be reduced below the environmental standard.

  An apparatus configuration example of the processing method of the present invention is shown in FIG. The illustrated processing system includes a first thickener 10 for washing complex heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, and means for adding a water-soluble sulfate as a reducing agent to the first thickener 10 (pipe). 11. Second thickener 12 for collecting cleaning waste water extracted from the first thickener 10, means (pipe) 13 for adding a precipitating agent and a flocculant to the second thickener, and processing waste water extracted from the second thickener 12. The third thickener 14 and a means (pipe) 15 for adding a precipitating agent and a flocculant to the third thickener 14 are provided, and discharge pipes for extracting starch from the bottoms of the second thickener 12 and the third thickener. 16 is connected.

  The composite heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium is supplied to the first thickener 10, and water and chemicals are added through the conduit 11 to be slurried and washed by stirring. After washing, the washing soil settled down on the tank bottom is extracted, and the washing wastewater remaining in the first thickener 10 is sent to the second thickener 12. In the second thickener 12, precipitation is performed such that sulfate ions are present 50 to 1000 times the selenate concentration and / or arsenate concentration, and the water-soluble barium salt is 1.5 to 10 times the sulfate ion concentration. Agents (water soluble sulfate and water soluble barium salt) are added through line 13. Further, ferric chloride is added through the pipe line 13 so as to have a concentration of 100 to 10,000 mg / L. In addition, the pH is adjusted to 5.5 to 10 to produce a starch containing selenium and / or arsenic together with iron hydroxide and barium. This starch is extracted out of the system through a discharge line 16 connected to the bottom of the tank.

On the other hand, the treated wastewater remaining in the second thickener 12 is sent to the third thickener 14. A precipitating agent (sodium sulfate, etc.) is added to the third thickener 14 through the line 15 so that the concentration of sulfate ions is 1.5 to 2 times the dissolved barium concentration, and a flocculant (ferric chloride) is added. And adjust the pH to 5.5-10 to produce a starch containing iron hydroxide and residual barium. This starch is extracted out of the system through a discharge line 16 connected to the bottom of the tank. On the other hand, the final treated wastewater remaining in the third thickener 14 is discharged outside the system after pH adjustment as necessary.

[Example 1]
In the illustrated processing system, 100 g of contaminated soil containing 80 mg / kg of hexavalent chromium, 180 mg / kg of selenium, and 50 mg / kg of arsenic is supplied to the first thickener, and 1000 g of water is added to form a slurry, to which 2.0 g of iron sulfate is added. Was added as an aqueous solution and stirred for 30 minutes at 300 rpm. After stirring, the washed soil was extracted from the tank bottom and separated from the washing waste water. Next, this washing wastewater was sent to the second thickener, and 8.8 g of barium chloride dihydrate was added to this washing wastewater, and stirred for 30 minutes while maintaining the pH at 5.5. Further, 0.8 mL of a 38% ferric chloride aqueous solution was added, and the mixture was stirred for 30 minutes while maintaining the pH at 9.0. After stirring, the starch precipitated on the bottom of the tank was extracted and separated from the waste water. On the other hand, the treated wastewater remaining in the second thickener was sent to the third thickener, and 8.5 g of sodium sulfate was added to the wastewater, followed by stirring for 10 minutes while maintaining the pH at 5.5. Further, 3.6 mL of a 38% aqueous solution of ferric chloride was added, and the mixture was stirred for 30 minutes while maintaining the pH at 7.0 to 8.0. After stirring, the starch at the bottom of the tank was extracted, and the concentrations of hexavalent chromium, selenium and arsenic contained in the waste water remaining in the third thickener were measured. The hexavalent chromium concentration was 005 mg / L or less, and the selenium concentration was 0.00. The arsenic concentration was 0.01 mg / L or less. The results are shown in Table 1.

[Example 2 and Example 3]
The contaminated soil was treated in the same manner as in Example 1 except that the treatment conditions shown in Table 1 were used. The results are shown in Table 1 together with the processing conditions.

[Comparative example]
The contaminated soil was treated in the same manner as in Example 1 except that the treatment conditions shown in Table 1 were used. The results are shown in Table 1 together with the processing conditions.

  As shown in Table 1, according to the treatment method (A1 to A3) of the present invention, the concentration of hexavalent chromium contained in the soil after washing is greatly reduced, and the concentrations of selenium and arsenic are also almost halved. Has been. In addition, the hexavalent chromium concentration, selenium concentration, arsenic concentration, and iron concentration in the final treatment wastewater are also below the environmental standard. On the other hand, Comparative Example B1 has a high hexavalent chromium concentration and arsenic concentration contained in the soil after washing, and the washing effect on hexavalent chromium and arsenic is insufficient. Further, in Comparative Example B2, the selenium concentration contained in the soil after washing is high, and in Comparative Example B3, the hexavalent chromium concentration contained in the soil after washing is high, and in any case, the washing effect on hexavalent chromium or arsenic is insufficient. is there.

Process drawing which shows the outline of the processing system of this invention

Explanation of symbols

  10-first thickener, 11-line, 12-second thickener, 13-line, 14-third thickener, 15-line, 16-discharge line.

Claims (6)

  For complex heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, as a first step, the contaminated soil is washed into a slurry, and this slurry contains water-soluble sulfuric acid of a metal having a lower standard electrode potential than hexavalent chromium. Salt is added to reduce hexavalent chromium and promote elution of selenium and / or arsenic. As a second step, water-soluble sulfate and water-soluble barium salt are added to the washing waste water after the first step. A starch containing selenium and / or arsenic together with iron hydroxide and barium is produced and separated, and further, as a third step, barium remaining by adding water-soluble sulfate to the treated waste water after the second step and A method for removing heavy metals from complex heavy metal contaminated soil, characterized in that iron is starched and separated.   In the first step, ferrous sulfate or stannous sulfate is used as a sulfate, and the pH of the contaminated soil slurry is adjusted to 4 to 9 to reduce chromium and elute selenium and / or arsenic. A method for removing heavy metals to be described.   In the second step, water-soluble sulfate is added so that sulfate ions are present 50 to 1000 times the selenate concentration and / or arsenate concentration, and then the water-soluble barium salt is added to a sulfate ion concentration of 1.5 to 1.5. 10 times, and ferric chloride was added to a concentration of 100 to 10000 mg / L, and the pH was adjusted to 5.5 to 10 together with iron hydroxide and barium. Alternatively, the heavy metal removal method according to claim 1, wherein a starch containing arsenic is produced and separated.   In the third step, sulfate ion of 1.5 to 2 times the dissolved barium concentration is added, ferric chloride is further added, pH is adjusted to 5.5-10, and iron hydroxide and residual barium are added. The method for removing heavy metals according to claim 1, 2 or 3, wherein the contained starch is produced and separated.   First thickener for cleaning complex heavy metal contaminated soil containing selenium and / or arsenic together with hexavalent chromium, means for adding water-soluble sulfate as a reducing agent to first thickener, and storing wastewater drained from first thickener 2 thickeners, means for adding precipitant and flocculant to the second thickener, third thickener for collecting treated waste water extracted from the second thickener, means for adding precipitant and flocculant to the third thickener, from each thickener An apparatus for removing heavy metals from composite heavy metal-contaminated soil, characterized by having a discharge conduit for extracting starch. The heavy metal removing apparatus according to claim 5, wherein an ultrasonic vibrator is provided at least in the first thickener.

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