JP2009148749A - Heavy metal-containing water treating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heavy metal-containing water treating system with simple and practically superior steps, and effectively removing even heavy metals forming a firm complex from drainage. <P>SOLUTION: The treating method includes steps of: adding a reducing iron compound to heavy metal-containing water; forming precipitation by leading the heavy metal-containing water added with the reducing iron compound to a reaction vessel; separating the formed precipitation (sludge) into solid matter and liquid; and returning all or part of the separated sludge to the reaction vessel by turning it to alkalinity. The returned sludge is adjusted to pH of 11-13, the reaction vessel is adjusted to pH of 8.5 or more, and the reducing iron compound precipitation having green rust and iron ferrite as a main component is made to take in heavy metals in a sealed non-oxidation environment for precipitation. The treating method has a step of decomposing the heavy metal complex by adding hydrogen peroxide to the heavy metal-containing water and by Fenton oxidation in the presence of a ferrous compound, prior to the adding step of the iron compound. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an economical treatment system capable of efficiently removing heavy metals forming a strong complex from waste water or the like. More specifically, the present invention is a process for treating heavy metal-containing water that is simple in process, excellent in practicality, and capable of efficiently removing heavy metals forming a strong complex from waste water or the like. About.

  A processing method is known in which divalent iron ions are added to wastewater containing heavy metals, and then the temperature of the liquid is maintained at 30 ° C. or higher, in an air-blocking environment, and alkali is added to generate and remove starch. (Patent Document 1). In addition, alkali is added to wastewater containing heavy metals to precipitate heavy metal hydroxides, inert gas is introduced into this treatment liquid to remove dissolved oxygen, and then ferrous salt is added in the alkaline region. There is known a processing method for precipitating heavy metals and then blowing air into the iron-containing precipitate to precipitate the heavy metals remaining in the liquid (Patent Document 2). In addition, ferrous hydroxide and alkali are added to the wastewater to precipitate heavy metals, while a part of the sludge is circulated to the reaction tank after alkali addition to increase the processing efficiency. (Patent Document 3).

  However, it is difficult to reduce the concentration of heavy metals in the waste water to an environmental standard value of 0.01 mg / L or less in any of the above conventional treatment methods. Further, in the method of simply adding ferrous hydroxide, oxygen in the wastewater reacts with ferrous ions, so it is necessary to remove dissolved oxygen in the wastewater in advance, and the treatment process is troublesome. Furthermore, since precipitation of ferrous hydroxide has a large water content and becomes bulky, the burden on slurry processing is large as it is.

In addition, ferrous ions etc. are added to heavy metal wastewater, adjusted to pH 5 or higher to produce iron ferrite or pseudo iron ferrite, and the generated ferrite sludge is separated into solid and liquid, and part of it is returned to the reaction tank And the processing method which removes heavy metals from waste water by circulating sludge is known (patent document 4). This method pays attention to the fact that ferrite sludge (FeO · Fe ₂ O ₃ ) contains ferrous iron and ferric iron. Ferrite sludge is easier to form with ferrous iron and ferric iron than ferrous alone. However, the ferrite sludge of this treatment method has a weak reducing power, and even if this sludge is returned to the reaction tank, the effect of removing heavy metals is limited.

  Furthermore, in the wastewater treatment method of adding sludge by adding alkali to water containing heavy metals, separating the sludge, adding alkali to a part of the separated sludge and returning this alkali sludge to the reaction tank Methods are known (Patent Documents 5 and 6). However, it is difficult to reduce heavy metals below the environmental standard value with alkaline sludge alone.

Moreover, the processing method using Fenton reaction is known as a method of processing organic waste water. This method is a method in which hydrogen peroxide is added in the presence of ferrous ions to generate hydroxy radicals, and organic substances are decomposed by this oxidizing power (Patent Document 7, etc.). Even so, it is difficult to reduce heavy metals below the environmental standard.
Japanese Patent Laid-Open No. 08-267076 JP 2002-326090 A JP 2001-9467 A Japanese Patent Laid-Open No. 2001-321781 Japanese Patent Publication No. 61-156 Japanese Patent Laid-Open No. 05-57292 (Japanese Patent No. 2910346) JP 2004-181446 A Japanese Patent No. 395978

  A treatment method for heavy metal-containing water that solves the above problems by improving a treatment method based on the conventional ferrite method is known (Patent Document 8: Japanese Patent No. 395978). In this treatment method, a precipitate containing green rust and iron ferrite is formed, so the precipitate is consolidated, solid-liquid separation is good, and ferrite treatment is possible at room temperature, and the concentration of heavy metals is the environmental standard value. It has the advantage that it can be reduced to 0.01 mg / L or less.

  In the above treatment method for removing heavy metals by forming a precipitate containing green rust and iron ferrite, the heavy metals contained in the waste water and the like react with a chelating agent or an organic acid to form a strong complex. There was a tendency that the removal effect of heavy metals did not improve. The present invention improves this point and provides a treatment method that is excellent in the effect of removing heavy metals even when heavy metals contained in waste water or the like form a strong complex.

The present invention relates to a method for treating heavy metal-containing water in which the above-described problems are solved by the configurations shown in the following [1] to [4].
[1] A step of adding a reducing iron compound to heavy metal-containing water [iron compound addition step], a step of introducing a heavy metal-containing water added with a reducing iron compound into a reaction vessel to generate a precipitate [precipitation step] , A process for solid-liquid separation of the generated precipitate (sludge) (solid-liquid separation process), a process for converting all or part of the separated sludge to alkalinity and returning it to the reaction tank (sludge return process). The sludge to be returned is adjusted to pH 11 to 13, the reaction tank in the precipitation step is adjusted to alkaline of pH 8.5 or more, and a reducing iron compound mainly composed of green last and iron ferrite in a sealed non-oxidizing atmosphere. In the treatment method of forming a precipitate, incorporating heavy metals into the iron compound precipitate and precipitating, and removing the heavy metals by solid-liquid separation of the precipitate, prior to the iron compound addition step, Of iron compounds A method for treating heavy metal-containing water comprising adding hydrogen peroxide in the presence to decompose a heavy metal complex by Fenton oxidation [Fenton oxidation step] and introducing the treated heavy metal-containing water into the iron compound addition step .
[2] After the Fenton oxidation step, a reducing agent is added to remove hydrogen peroxide residues [hydrogen peroxide removal step], and after this treatment, heavy metal-containing water is introduced into the iron compound addition step. The processing method of the heavy metal containing water described.
[3] In the treatment method of [2] above, using a reducing agent that removes hydrogen peroxide remaining in the Fenton oxidation step and ferrous sulfate as the reducing iron compound in the iron compound addition step, the hydrogen peroxide removal step And a method for treating heavy metal-containing water in which the iron compound addition step is combined.
[4] The method for treating heavy metal-containing water according to any one of [1] to [3] above, wherein the oxidative decomposition tank in the Fenton oxidation step is adjusted to pH 2 to 5 for treatment.

  In the treatment method of the present invention, since the ferrous compound and hydrogen peroxide are added to the heavy metal-containing water and the heavy metal complex is decomposed by Fenton oxidation prior to the iron compound addition step, the heavy metal contained in the wastewater is chelated. Even if it reacts with an agent or an organic acid to form a strong complex, this heavy metal complex is oxidatively decomposed and then introduced into a precipitation process that includes green last and iron ferrite. Can be increased.

Hereinafter, the processing method of the present invention will be specifically described based on embodiments.
The treatment method of the present invention includes a step of adding a reductive iron compound to heavy metal-containing water [iron compound addition step], a step of introducing a heavy metal-containing water to which the reductive iron compound has been added to a reaction vessel to generate a precipitate [ (Precipitation step), solid-liquid separation of the generated precipitate (sludge) (solid-liquid separation step), and a step (sludge return step) of returning all of the separated sludge to alkalinity and returning it to the reaction tank The sludge to be returned to the reaction tank is adjusted to pH 11 to 13, the reaction tank in the precipitation step is adjusted to alkaline of pH 8.5 or higher, and the reduction mainly consists of green last and iron ferrite in a sealed non-oxidizing atmosphere. In the processing method in which a heavy iron compound precipitate is formed, heavy metal is incorporated into the iron compound precipitate to be precipitated, and this precipitate is solid-liquid separated to remove heavy metals, the heavy compound is contained prior to the iron compound addition step. in water 1 Heavy metals characterized by adding hydrogen peroxide in the presence of an iron compound to decompose heavy metal complexes by Fenton oxidation [Fenton oxidation step], and introducing the treated heavy metal-containing water into the iron compound addition step It is a processing method of contained water.

  In the present invention, heavy metal-containing water broadly means water containing heavy metals, and includes various wastewater and wastewater generated naturally and artificially, such as factory wastewater and sewage, seawater, river water, This refers to water from marshes and lakes, surface pools, rivers and other dams, underground running water and pools, underdrains, etc. that contain heavy metals. In the following description, these waters may be referred to as waste water, and the heavy metal containing water may be referred to as waste water containing heavy metals.

  In the present invention, heavy metals refer to heavy metal elements such as selenium, cadmium, hexavalent chromium, lead, zinc, copper, nickel, arsenic, and antimony, and metal elements. The treatment system of the present invention has an excellent removal effect with respect to any one or more of the heavy metals that are the sources of contamination contained in the waste water and the like.

  In the present invention, the pH may be measured by the pH measurement method based on the glass electrode method of JIS standard (JIS K0102 12.1).

  The outline of this processing system is shown in FIG. As shown in the figure, this treatment system includes a tank 10 for adding a reducing iron compound to heavy metal-containing water [iron compound addition step], and sealing a non-oxidizing atmosphere in which the heavy metal-containing water added with the reducing iron compound is reacted. Reaction tank 20 [precipitation step], means 30 for solid-liquid separation of the slurry extracted from the reaction tank 20 [solid-liquid separation step], tank 40 for adding alkali to the separated sludge, and returning alkaline sludge to the reaction tank 20 A circulation processing system including a pipe line [sludge returning step] and a pipe line communicating these tanks and the solid-liquid separation means is formed.

  Prior to the iron compound addition step, the treatment system of the present invention further includes an oxidative decomposition tank 50 [Fenton, which decomposes heavy metal complexes by Fenton oxidation by adding hydrogen peroxide to heavy metal-containing water in the presence of a ferrous compound. Oxidation process] is provided. The ferrous compound may be added during the introduction of waste water or the like into the oxidative decomposition tank 50, and hydrogen peroxide may be added after the ferrous compound is added to the oxidative decomposition tank 50.

  The oxidative decomposition tank 50 in the Fenton oxidation process is adjusted to pH 2-5, preferably pH 3-4. If the pH rises during the reaction, sulfuric acid or hydrochloric acid may be added to lower the pH.

In the oxidative decomposition tank 50, when hydrogen peroxide [H ₂ O ₂ ] is added in the presence of ferrous ions [Fe ²⁺ ], the hydrogen peroxide is converted into hydroxyl radicals [HO ⁺ ] and hydroxy ions [OH ⁻ ]. A Fenton reaction that decomposes into a heavy metal is produced, and the complex of heavy metals is decomposed by this hydroxy radical [HO ⁺ ]. Since the oxidizing power of hydroxy radicals is strong, even when heavy metals contained in waste water or the like react with a chelating agent or an organic acid to form a strong complex, this heavy metal complex can be oxidatively decomposed. Furthermore, when organic matter is contained in the waste water, this organic matter is also decomposed.

Further, in the treatment system of FIG. 1, a hydrogen peroxide removal tank 51 (hydrogen peroxide removal process) for removing residual hydrogen peroxide is provided after the oxidative decomposition tank 50. The removal tank 51 is added with a reducing agent to remove residual hydrogen peroxide. As the reducing agent, sulfite ions [SO ₃ ²⁻ ], oxalic acid, ascorbic acid, iron sulfate, iron powder and the like can be used. Residual hydrogen peroxide reacts with these reducing agents and is decomposed.

When hydroxy radicals are generated by the Fenton reaction, ferrous ions [Fe ²⁺ ] are oxidized to ferric ions [Fe ³⁺ ], and precipitation of ferric hydroxide occurs. Further, ferric hydroxide precipitate is formed in the hydrogen peroxide removal step. Since these precipitates are incorporated into the subsequent green last / iron flight precipitation, they need not be removed in the oxidative decomposition step and the hydrogen peroxide removal step. If the adsorbate of these precipitates damages the subsequent green last / iron flight precipitate, the precipitate is removed.

  Ferrous sulfate can be used as a reducing agent for removing residual hydrogen peroxide, and this can also be used as a reducing iron compound to be added to waste water or the like in the next step. In this case, as shown in FIG. 2, the removal tank 51 is omitted, and heavy metal containing wastewater treated in the oxidative decomposition tank 50 is directly introduced into the addition tank 10, and residual peroxidation contained in the wastewater etc. What is necessary is just to increase and add the quantity of ferrous sulfate which carries out hydrogenolysis.

In the treatment system of the present invention, the heavy metal-containing water is guided to the addition tank 10 and the reducing iron compound is added. As the reducing iron compound, ferrous compounds such as ferrous sulfate (FeSO ₄ ) and ferrous chloride (FeCl ₂ ) can be used. The addition amount of the ferrous compound is appropriate such that the Fe ²⁺ ion concentration is 400 to 600 mg / L. Heavy metal-containing water to which the reducing iron compound is added is introduced into the reaction tank 20.

  The alkaline sludge is returned to the reaction tank 20 from the solid-liquid separation step together with the heavy metal-containing water to which the reducing iron compound is added, and mixed with the heavy metal-containing water. This alkaline sludge is adjusted to pH 11 to 13 by adding alkali to a part or all of the precipitate (sludge) separated into solid and liquid in the subsequent step. As the alkaline substance to be added, slaked lime, quick lime, sodium hydroxide or the like can be used. By mixing alkaline sludge, the pH of the reaction tank 20 is adjusted to 8.5 to 11, preferably 9.0 to 10.

  In the reaction vessel 20, heavy metal-containing water to which a reducing iron compound is added and alkaline return sludge are mixed and reacted in a non-oxidizing atmosphere to generate a reducing iron compound precipitate. This iron compound precipitate is a mixture of green last and iron ferrite and is a reductive precipitate.

Green last is a blue-green substance in which a hydroxide of ferrous iron and ferric iron forms a layer, and has a structure in which an anion of heavy metals is incorporated between layers, and is represented by, for example, the following formula (1).
_{^{[Fe II (6-x) Fe}} III x (OH) 12 ] _x + [A _x / n · yH ₂ O] _x ^- ... (1)
(0.9 <x <4.2, Fe ²⁺ / total Fe = 0.3 to 0.85)

Further, iron ferrite is an iron salt of Fe ^II , which is mainly composed of magnetite (Fe ^II Fe ^III ₃ O ₄ ), but may contain a heavy metal ferrate in part. In the reducing iron compound precipitate of the present invention, for example, heavy metal ions in water containing heavy metals are taken in between the layers of the green last, and iron ferrite is formed in a state in which the heavy metals are partially included. Specifically, for example, hexavalent selenium (SeO ₄ ^2- ) contained in waste water and the like is reduced by ferrous compounds to tetravalent selenium (SeO ₃ ^2- ) and elemental selenium, which are It precipitates in the state of being taken in between the layers.

  The treatment method of the present invention uses a closed reaction tank in which the inflow of air is blocked in order to produce the reductive iron compound precipitate in the reaction tank 20, and has a pH of 8.5 to 11, preferably pH 9 in a non-oxidizing atmosphere. The reaction is carried out under alkaline conditions of 0.0 to 10. The liquid temperature may be about 10 ° C. to 30 ° C. and does not need to be heated. The reaction time may be about 30 minutes to 3 hours.

  Even in a treatment method in which ferrous compound and alkali are added to heavy metal-containing water to produce an iron compound precipitate, the reaction vessel is not sealed as in the conventional case, and the reaction is performed in a non-oxidizing atmosphere. Those having a low alkali level do not produce precipitates having the above reducing power, and the same effects as those of the present invention cannot be obtained.

In the processing method of the present invention, as described above iron compound precipitate comprising a mixture of green rust and iron ferrite has a reducing power, the ratio of divalent iron ion and total iron ions該沈lees [Fe ²⁺ / Fe (T )] Is preferably 0.4 to 0.8, and the iron ion ratio is more preferably controlled to 0.55 to 0.65. When this ratio is out of the above range, the reduction of heavy metals becomes insufficient, or the sedimentation property of starch deteriorates, which is not preferable. By producing the reducible iron compound precipitate, the contained heavy metals are reduced and easily incorporated into the precipitate.

As for the above [Fe ²⁺ / T-Fe] measurement, the [T-Fe] concentration was measured by “1,10 phenanthroline absorptiometry” of JIS standard (JIS K0102 57.1) after acid precipitation of iron compound precipitate. Similarly, the [Fe ²⁺ ] concentration may be measured by the phenanthroline spectrophotometry method without adding a reducing agent. [Fe ²⁺ / T-Fe] is calculated from the ratio of the absorbances of the two.

By repeating the return of the alkaline sludge to the reaction tank 20 and repeating the reaction with the heavy metal-containing water to which the reducing iron compound is added, the green rust is oxidized and iron ferrite is formed. Gradually turns black. Since the reducibility is lost when most of the green rust becomes iron ferrite, in the treatment method of the present invention, the ratio [Fe ²⁺ / Fe (T)] of the divalent iron ions to the total iron ions in the iron compound precipitate is set as described above. Control within the range to produce a reducing precipitate.

  In the treatment method of the present invention, the reducing sludge (iron compound precipitation) is separated, and a part or all of the reduced sludge is alkalinized and returned to the reaction tank, reacted in a non-oxidizing atmosphere, and the reducing sludge is precipitated again. By repeating this process, iron ferrite is formed while maintaining the reducibility of the sludge (precipitation), so that the consolidation of the precipitation proceeds and the concentration of the starch is remarkably increased, thereby improving the effect of removing heavy metals. Incidentally, precipitation (sludge) mainly composed of iron hydroxide is bulky and has a heavy dehydration burden. Further, in the treatment method of the present invention, the iron ferrite forming the precipitate is magnetized because it is mainly composed of magnetite, and the separated precipitate can be adsorbed to a magnet for treatment.

The slurry discharged from the reaction tank 20 is guided to a solid-liquid separation means such as a thickener, for example, and the sludge is settled on the tank bottom and separated. This starch can be solid-liquid separated to remove heavy metals out of the system. Moreover, as already stated, an alkali is added to some or all of the sludge to adjust to pH 11 to 13 and returned to the reaction tank 20, and the precipitation generation reaction is repeated in the reaction tank 20. The ratio of sludge to be returned (circulation ratio of the returned sludge) is determined so that the ratio [Fe ²⁺ / Fe (T)] of the divalent iron ions and total iron ions in the precipitate generated in the reaction tank 20 is within the above range. Just do it. The treatment method of the present invention can be carried out in either a batch type or a continuous type.

  Further, as described above, all or part of the sludge separated in the solid-liquid separation means is made alkaline and returned to the reaction tank, but the sludge not returned to the reaction tank is filtered and dehydrated by a filter press or the like, Drains out of the system. On the other hand, the filter residue still has a reducing power, and the filter residue has good water permeability. Therefore, as shown in FIG. The contamination contained in the drainage can be decomposed and removed from the drainage by utilizing the reducing power remaining in the residue.

  According to the treatment method of the present invention, the concentration of heavy metals contained in waste water or the like can be reduced to 0.01 mg / L or less of the waste water reference value. Furthermore, this treatment method does not require heating, can be ferritized at room temperature, forms a compact compact starch, and has a very good dehydration, high removal effect of heavy metals, and economic efficiency. In addition, it is a processing method with excellent handleability.

  Hereinafter, the present invention will be specifically described by Examples and Comparative Examples. In addition, the ferrite of starch was determined by magnetism, and the compactness of starch was determined by bulkiness and sedimentation. For heavy metal measurement methods, Cd is JIS standard (JIS K0102 55.3), Pb is JIS standard (JIS K0102 54.3), Cu is JIS standard (JIS K0102 52.4), Zn is JIS standard (JIS K0102 53.3), Cr (VI) was measured by ICP emission spectroscopic analysis of JIS standard (JIS K0102 65.2.4). As was measured by the hydrogen compound generation atomic absorption method of JIS standard (JIS K0102 61.2).

〔Example〕
According to the treatment flow of the present invention shown in FIG. 1, wastewater containing heavy metals and the chelating agent ethylenediaminetetraacetic acid (EDTA) was treated in a batch manner as follows. First, 2.0 L of the waste water (concentration of Cd, Pb, Cu, Zn, As, and Cr (VI): 2 mg / L and EDTA: 75 mg / L, respectively) is introduced into the oxidative decomposition tank 50 and ferrous sulfate. Was added as Fe (II) to 50 mg / L, and 13 mL of hydrogen peroxide having a concentration of 3.5% by mass was added thereto over 2 hours. This waste water is introduced into the removal tank 51 and ferrous sulfate 100 mg / L is added to decompose and remove residual hydrogen peroxide, and then introduced into the addition tank 10 so that the ferrous sulfate is 500 mg / L. Added. Wastewater added with ferrous sulfate in the addition tank 10 was introduced into the reaction tank 20 and reacted for 20 minutes to form a precipitate. Next, the slurry extracted from the reaction vessel 20 was allowed to stand for 18 hours with a thickener to settle the precipitate, and solid-liquid separation was performed. The total amount of the solid-liquid separated precipitate was introduced into the alkali addition tank 40, and the one adjusted to pH 11 to 13 by adding 25% by mass of caustic soda was returned to the reaction tank 20 to repeat the formation and separation of the precipitate. Table 1 shows a comparison of treatment results of heavy metals with and without the Fenton oxidative decomposition process.

  As shown in Table 1, when compared with the treatment method of the present invention having the Fenton oxidation step and the treatment method of the comparative example without the Feton oxidation step, both As and Cr (VI) not forming an EDTA complex were treated. The concentration in the later effluent was less than 0.01 mg / L, a similar concentration. On the other hand, for Cd, Pb, and Zn that easily form a complex with EDTA, the treatment concentration was significantly reduced by the Fenton oxidation in the treatment method of the present invention. Compared with the treatment method of the comparative example, in particular, Cd improved the removal rate from 20% to 96%, and Pb improved from the removal rate from 70% to 99.5% or more, and water quality lower than the drainage standard was obtained. The Fenton oxidation step was preferably adjusted to a pH range of 2 to 5, and it was confirmed by experiments that a pH range of 3 to 4 was more preferable.

Process chart of the present invention Other process steps according to the present invention

Explanation of symbols

10-addition tank, 20-reaction tank, 30-solid-liquid separation tank, 40-alkali addition tank, 50-oxidative decomposition tank, 51-residual hydrogen peroxide removal tank.

Claims (4)

A step of adding a reducing iron compound to heavy metal-containing water [iron compound addition step], a step of introducing heavy metal-containing water added with a reducing iron compound into a reaction vessel to generate a precipitate [precipitation step], Sludge that has a step of solid-liquid separation of the precipitate (sludge) (solid-liquid separation step), a step of returning all of the separated sludge to alkalinity and returning it to the reaction tank (sludge return process) and returning it to the reaction tank Is adjusted to pH 11-13, the reaction tank in the precipitation step is adjusted to alkaline of pH 8.5 or more, and a reductive iron compound precipitate mainly composed of green last and iron ferrite is produced in a sealed non-oxidizing atmosphere. In the treatment method in which heavy metal is taken into the iron compound precipitate to be precipitated, and this precipitate is solid-liquid separated to remove heavy metal, prior to the iron compound addition step, the ferrous compound is added to the heavy metal-containing water. In the presence Adding hydrogen peroxide to decompose heavy metals complex by Fenton oxidation [Fenton oxidation step] The method for treating heavy metals containing water to the treated heavy metals containing water and introducing the iron compounds addition step.
The heavy metal according to claim 1, wherein a reducing agent is added after the Fenton oxidation step to remove the hydrogen peroxide residue [hydrogen peroxide removal step], and the heavy metal-containing water is introduced into the iron compound addition step after this treatment. Treatment method of contained water.
3. The treatment method according to claim 2, wherein the reducing agent for removing hydrogen peroxide remaining in the Fenton oxidation step, and ferrous sulfate as the reducing iron compound in the iron compound addition step, the hydrogen peroxide removal step and the iron compound addition A method for treating heavy metal-containing water that is also used in the process.
The processing method of the heavy metal containing water in any one of Claims 1-3 which adjusts the oxidative decomposition tank of a Fenton oxidation process to pH 2-5, and processes.

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