JP4757457B2 - Soil purification treatment method and soil purification treatment apparatus - Google Patents

Description

  The present invention relates to a contaminated soil purification treatment method and a soil purification treatment apparatus suitable for purifying soil contaminated with nonvolatile hydrophobic organic compounds such as PCBs and dioxins contained in the soil.

  Conventionally, in an era when the toxicity was not recognized, a complete waste liquid treatment as in the present situation was not performed on chemical substances used in factories and the like. For this reason, there are soils contaminated with non-volatile hydrophobic organic compounds (hereinafter referred to as PCB, etc.) represented by PCBs and dioxins, etc. in the factory site, and the purification process is a problem. It has become to. On the other hand, when the soil contaminated with PCB or the like is subjected to purification treatment, PCB or the like is scattered and mixed in the soil, so it must be treated with a large amount of soil, and a large-scale treatment is required. Not that. Currently known methods include, for example, a thermal decomposition method, an alkali catalytic chemical decomposition method, a melt solidification method, and the like. However, none of these are simple and reliable treatment methods, and development of a system that can be purified economically so that contaminated soil can be purified so as to meet safety standards, and the number of treatment steps is small. Long-awaited.

On the other hand, the present applicant has already proposed a simple and reliable treatment method capable of purifying contaminated soil so as to satisfy safety standards (see Patent Document 1).
However, as a result of further studies by the present inventors, the above-described method uses a metal ion such as iron as a catalyst, so the catalyst remains in the treated soil, and there is a problem depending on the use of the treated soil. There was sometimes. Further, in order to treat soil, a large amount of catalyst is required, and there is a problem in terms of treatment cost.

JP 2000-80732 A

  Accordingly, an object of the present invention is to reliably and more economically treat soil contaminated with a non-volatile hydrophobic organic compound such as PCB, and to recycle the treated soil. Another object of the present invention is to provide a method for purifying contaminated soil without any problems, and a soil purification apparatus capable of applying the method to actual soil treatment.

The above object is achieved by the present invention described below. That is, the present invention is a method for purifying contaminated soil for removing at least one of PCB and dioxins from soil by oxidizing and detoxifying them so that they can be reused. The slurry is adjusted to a pH of 4 or less , hydrogen peroxide is added , the slurry is filled in a pressure resistant container, the container temperature is 150 to 200 ° C., and the container pressure is adjusted. under conditions and 0.5 to 10 MPa, at least one of the PCB and dioxins are dissolved in water, without the use of a catalyst, oxidative decomposition by hydroxyl radicals generated from hydrogen peroxide (oxygen-containing gas as the oxidant A soil remediation treatment method characterized in that oxidative decomposition is used). As a more preferable embodiment, there is a soil purification method in which the ratio of soil to water in the slurry is soil: water = 1: 10 to 1: 2 in terms of weight.

Another embodiment of the present invention is an apparatus for purifying contaminated soil that can be oxidatively decomposed to remove at least one of PCBs and dioxins from the soil, detoxify them, and reusable. A slurry tank for mixing the slurry to adjust the pH of the slurry to 4 or less, at least one line mixer for mixing hydrogen peroxide with the slurry, and a slurry sent from the line mixer and a reaction vessel of the plurality of pressure resistance to be filled, the container is in a state filled with the slurry, temperature inside the container 150 to 200 ° C., under the conditions of container internal pressure 0.5 to 10 MPa, the PCB and dioxin at least one of s are dissolved in water, without the use of a catalyst, oxygen-containing as oxidative decomposition (oxidizing agent with a hydroxy radicals generated from hydrogen peroxide It has a structure capable of excluding oxidative decomposition using gas) and is configured to perform the above-mentioned treatment sequentially in multiple stages in a plurality of reaction vessels. Device.

  There is provided a purification method for contaminated soil that can reliably and more economically treat soil contaminated with non-volatile hydrophobic organic compounds such as PCB, and can use the treated soil without any problems. Provided.

  Hereinafter, the present invention will be described in more detail with reference to the best mode for carrying out the invention. As a result of intensive studies to solve the above-mentioned problems of the prior art, the present inventors have added water to the contaminated soil and mixed to form a slurry, and the slurry is heated and pressurized, and a catalyst is used. Thus, the present inventors have found that PCBs and the like in soil can be satisfactorily decomposed by oxidative decomposition treatment with an oxidizing agent, particularly hydrogen peroxide, without damaging them.

  OH radicals (hydroxy radicals) generated by a decomposition reaction of hydrogen peroxide or the like are generated under normal temperature and warm (about 40 to 60 ° C.) conditions, but the generation efficiency is low. For this reason, usually a metal catalyst is used in combination. On the other hand, according to the study by the present inventors, it was found that the decomposition efficiency is remarkably improved by heating and pressurizing, and the OH radical generation efficiency can be improved without using a catalyst. Furthermore, as a result of investigating the purification treatment for soil contaminated with nonvolatile hydrophobic organic compounds such as PCB in a system using hydrogen peroxide as an oxidant, the temperature in the container was set to 100 to 250 ° C. It has been found that PCBs and the like in soil can be decomposed with hydrogen peroxide without using a catalyst if treated under conditions of pressure of 0.5 to 10 MPa. The present inventors consider that this reason is as follows.

  The wet oxidation radical method is a technique for decomposing harmful chemical substances eluted in water under heating and pressurization using the strong oxidizing power of OH radicals. When water is heated and pressurized, as shown in FIG. 1, the dielectric constant decreases with increasing temperature. This dielectric constant correlates with the polarity of water, and as the dielectric constant decreases, the polarity of water also decreases, and it is known that water exhibits properties similar to those of solvents. Water is in a state close to that of methanol at 200 ° C., ethanol at 250 ° C., and acetone at 300 ° C., and the solubility in organic matter is improved compared to water at normal temperature. In the temperature range of 100 to 250 ° C. in the present invention, the organic substance solubility is equivalent to that of methanol and ethanol.

  Here, when the soil slurry is heated and pressurized, the organic matter solubility of water is improved, and the organic matter is dissolved in water. When the pollutant is PCB or dioxins, PCB adsorbed on the soil surface dissolves in water when the contamination concentration is relatively low. On the other hand, organic matter components in the soil are mainly composed of macromolecular compounds such as biological carcasses, biological metabolites, and humic substances, and some of them are dissolved in water in the temperature range of 100 to 250 ° C. However, most of them are present without being dissolved.

  Here, the OH radical is a radical having extremely high reactivity, but its lifetime is several tens of ns and the diffusion distance is several tens of nm although it depends on the concentration of the reaction substrate. Also, OH radicals have low reaction selectivity and react immediately with most substances. For this reason, it is thought that it reacts preferentially with the substance dissolved in water. From the above, in the temperature range of 100 to 250 ° C., preferably 150 to 200 ° C., harmful chemical substances in soil (dissolved in water) by OH radicals without being disturbed by organic components in the soil. Will be decomposed preferentially.

  As a result of the above, if the conditions defined in the present invention are used, it is possible to remove PCBs and the like contained in trace amounts in the soil to a very low concentration by the wet oxidation radical method utilizing the characteristics of water and OH radicals. It is considered to be.

  On the other hand, if the treatment temperature is higher than 250 ° C., the heating energy becomes high, which is not economical. For example, if the treatment temperature is 300 ° C., the solubility of organic matter in water is comparable to that of acetone. For this reason, organic matter in the soil is also considerably dissolved in water, so that most of the OH radicals are consumed in the decomposition reaction of the soil organic matter present in large quantities in the water. Therefore, in this case, the removal rate of PCBs and dioxins contained in trace amounts in the soil is significantly reduced.

  In the purification treatment of contaminated soil with PCB or the like, it is effective to perform the following series of treatment methods under the above-described conditions. FIG. 2 schematically shows a system for realizing the soil purification treatment method of the present invention in which soil purification treatment is performed by oxidative decomposition with an oxidizing agent under heating and pressure. First, prior to the purification treatment, it is preferable to pre-treat the soil to be treated as necessary. As pretreatment, after crushing stones, concrete blocks, rocks, etc. included in the excavated soil, trash and other foreign substances, pebbles and sand are removed using a sieve and earth and sand separator. It is preferable to do. As an apparatus for pretreatment, for example, a hurricane (trade name, manufactured by Shinroku Seiki Co., Ltd.) for crushing stones, concrete blocks, and the like contained in soil can be suitably used. If such an apparatus is used, stones and the like can be crushed, peptized, and ground according to their use, so that these materials separated from the soil can be effectively used. Furthermore, it is also preferable to make the soil finer to 75 μm or less after removing impurities and the like as described above. In this way, the decomposition efficiency of PCB or the like can be further improved.

  In the method for purifying contaminated soil of the present invention, first, as shown in FIG. 2, using the slurry tank 4, water is added to the soil that has been pretreated as described above as necessary, and the agitator 6 is added. Mix and stir to form a slurry. In this case, the ratio of soil to water is preferably weight: soil: water = 1: 10 to 1: 2. At this time, a water-soluble organic solvent such as ethanol or methanol may be added. In this way, it is possible to further improve the oxidative decomposition efficiency of PCB and the like performed thereafter.

  Further, in the method for purifying contaminated soil according to the present invention, the pH of the soil S which has been made into a slurry by adding water as described above is adjusted to 4 or less, more preferably to 3 or less, and in this state, oxidation with an oxidizing agent is performed. Process. As the pH adjuster used in this case, it is preferable to use sulfuric acid, nitric acid or the like.

  Next, the slurry S prepared as described above is filled into a reaction vessel composed of a pressure vessel capable of bringing the inside indicated by 1 in the figure into a pressurized state, and the vessel of the reaction vessel 1 is filled. Under the conditions of the inner temperature of 100 to 250 ° C. and the pressure of 0.5 to 10 MPa, the PCB in the soil is oxidatively decomposed with an oxidizing agent without using a catalyst. The oxidative decomposition is performed as follows.

  In the present invention, the slurry S is heated to 100 to 250 ° C. and treated under a pressurized condition of 0.5 to 10 MPa. What is necessary is just to determine the shape or capacity | capacitance of the reaction tank 1 suitably according to the quantity of the soil etc. to process. 2 in the figure is a heater for heating the inside of the reaction tank 1. By heating the inside of the reaction tank 1 filled with the slurry by the heater 2, the inside of the reaction tank 1 is in a pressurized state. At this time, the temperature condition and the pressurizing condition are controlled by measuring the temperature in the reaction tank 1 with a thermometer provided in the reaction tank 1.

  In the present invention, the temperature in the reaction vessel 1 (more specifically, the temperature of the slurry) is set to 100 ° C. to 250 ° C., more preferably 150 to 200 ° C. as described above. Furthermore, the pressure in the pressure vessel constituting the reaction tank 1 is set to a pressure state in the range of 0.5 to 10 MPa, and more preferably in the range of 1.0 to 2.5 MPa. Moreover, it is preferable to add the oxidizing agent 3 in the reaction tank 1 and perform the treatment while stirring the slurry S filled in the reaction tank 1 with a stirrer.

  In the present invention, examples of the oxidizing agent used in the oxidation treatment include oxidizing agents used in conventionally known chemical oxidation methods such as hydrogen peroxide, calcium peroxide, ammonium persulfate, alkyl hydroperoxides, peroxide esters, peroxides, and the like. Although dialkyl oxide or diacyl oxide can be used, hydrogen peroxide is most preferable from the viewpoint of cost and by-products. The amount of the oxidizing agent such as hydrogen peroxide is not particularly limited and varies depending on the properties of the slurry to be treated. A preferable amount used is 0.05% of 30% hydrogen peroxide with respect to 1 g (dry) of soil. It is the range of -1.0g. Hereinafter, hydrogen peroxide will be described as a representative example.

  The oxidizing agent 3 such as hydrogen peroxide is preferably added to the slurry immediately before the slurry S enters the reaction vessel 1. As a method of addition, a pump is used, and mixing with the slurry is preferably performed by using the line mixer 5 to uniformly mix the oxidant 3 with the entire slurry S.

  In the present invention, the purification treatment is performed in the range of 100 to 250 ° C., but when the treatment temperature is less than 100 ° C., oxidation takes time under oxidation conditions that do not use a catalyst, and the efficiency is insufficient. In addition, the utilization efficiency of hydrogen peroxide is insufficient. Further, when the temperature exceeds 250 ° C., the heating energy consumption increases, and at the same time, the processing efficiency with respect to PCB or the like decreases due to the reason described above, so that economical and effective processing cannot be performed. The oxidation reaction time varies depending on the temperature and pressure of the reaction vessel 1, the performance of the line mixer 5, and the like. When thorough mixing of the oxidizing agent is performed under the condition of 0 MPa, a reaction time of about 15 to 30 minutes is sufficient. According to the study by the present inventors, it was found that 95% by mass or more of the pollutant such as PCB can be decomposed under such conditions although it varies depending on the contamination concentration.

  The slurry after the oxidation treatment may be squeezed with a squeezing machine or the like, then neutralized to pH 6.5 to 7.5 by adding an alkali such as sodium hydroxide, and backfilled to the excavated place. After neutralization, the water may be squeezed again. The water squeezed out by the press may be used again for soil purification treatment.

  FIG. 3 is a schematic diagram of a soil purification treatment apparatus according to the present invention when the soil purification treatment method according to the present invention consisting of the basic treatment described above is applied to actual soil treatment. As shown in the figure, in the actual treatment, the reaction vessel 1 is constituted by a group of reaction vessels 1a composed of a plurality of pressure vessels, and the non-volatiles in the slurry are sequentially provided in multiple stages in these reaction vessels 1a. The hydrophobic organic compound can be oxidized and decomposed. More specifically, first, the soil is made into a slurry in the slurry tank 4. At this time, a pH adjuster is put in the slurry tank to adjust the pH of the slurry S to 4 or less. The slurry S is sent from the slurry tank 4 to the reaction tank 1 by a pump, and a line mixer 5 is provided in the vicinity of the inlet of the reaction vessel 1a in the middle of the slurry tank. Mix. The temperature and pressure of the reaction vessel 1a are appropriately adjusted by the heater 2 having a temperature adjusting function. After conducting the oxidative decomposition reaction in one reaction vessel 1a, the slurry S is sent to the next reaction vessel 1a, and at that time, an oxidizing agent such as hydrogen peroxide is added in the same manner as described above.

  Thus, the slurry after the multi-stage processing is sent to the discharge tank by the pipeline, and the final processing is performed. At that time, if the pipeline passes through the slurry tank 4, the temperature of the soil heated in the reaction tank 1 can be lowered, and at the same time, the temperature of the slurry to be treated can be increased. The thermal energy used for processing can be used effectively. The treated slurry sent to the discharge tank by the pipeline as described above is discharged after the gas component is treated by activated carbon adsorption. Others are sent to the precipitation tank, and the supernatant water is discharged as treated water after adjusting the pH. Moreover, the precipitated solid content is backfilled as treated soil.

Hereinafter, the present invention will be described more specifically with reference to examples.
<Example 1>
After pre-treating the soil contaminated with PCB to a particle size of 75 μm or less, 3.0 liters of water is added to 1.2 kg (dry) of the contaminated soil and mixed and stirred for 10 minutes. To make a slurry. When PCB in the used soil was measured by GC-ECD, the contamination concentration was 22.73 mg per 1 kg of dry soil.

  The slurry was adjusted to pH 2.5 with sulfuric acid, and then purified using hydrogen peroxide under heating and pressurization using an apparatus as shown in FIG. After putting 1 L of water in the pressure vessel, it was sealed and the temperature in the vessel was raised to 150 ° C. with a heater. Next, the slurry was continuously injected into the reaction tank 1 composed of a pressure vessel using a pump, and was processed in the range of 150 to 200 ° C. Hydrogen peroxide was added with a chemical pump just before the slurry entered the pressure vessel, and the slurry and hydrogen peroxide were mixed with a line mixer. When the pressure in the container at the time of reaction was measured, it was 0.5 to 2.0 MPa. As hydrogen peroxide, 30% was used. The amount used was 4% of the solid content in the slurry in terms of 100% hydrogen peroxide (adding 4 g of 100% hydrogen peroxide to 100 g of soil (dry)). means). The reaction was performed for 30 minutes under these conditions. The PCB concentration of the slurry oxidized as described above was reduced to 1.25 mg / kg in terms of dry dry matter. Furthermore, when the dissolution test was conducted and the PCB concentration in the eluate was measured, no PCB was detected.

<Example 2>
The PCB-contaminated soil that had been pretreated in the same manner as in Example 1 was purified. The PCB concentration of the soil used was 95.64 mg / kg. The treatment conditions were the same as in Example 1, and this treatment was repeated 3 times on the soil. The PCB concentration of the slurry after the treatment was reduced to 0.23 mg / kg in terms of dry dry matter. Further, the dissolution test was performed and the PCB concentration in the eluate was measured. As a result, no PCB was detected.

< Reference Example 1 >
The same PCB-contaminated soil as that treated in Example 1 was treated in the same manner as in Example 1 except that the container internal temperature was treated in a temperature range of 100 to 150 ° C. As a result, the PCB concentration after the treatment was reduced to 2.15 mg / kg in terms of dry dry matter.

< Reference Example 2 >
The same PCB contaminated soil as that treated in Example 1 was treated in the same manner as in Example 1 except that the container internal temperature was treated in the temperature range of 200 to 250 ° C. As a result, the PCB concentration after the treatment was reduced to 1.42 mg / kg in terms of dry dry matter.

<Example 5>
The purification treatment of the dioxin simulated contaminated soil that had been pretreated in the same manner as in Example 1 was performed. As the simulated contaminated soil, hexane in which dioxins were dissolved was mixed with the soil, dried at room temperature for 2 days, and further stored in a stainless steel container for 2 weeks or more. The concentration of dioxins in the soil used was 4,000 ng / g (note: content rather than TEQ). The treatment conditions were the same as in Example 1, and this treatment was repeated twice on the soil. The dioxin concentration after the treatment was reduced to 11 ng / g in terms of dry dry matter.

<Comparative Example 1>
The same PCB-contaminated soil treated in Example 1 was performed in the same manner as in Example 1 except that the iron catalyst was added to the slurry. As an iron catalyst, Fe (OH) ₃ was added so that the iron ion concentration in the slurry at the beginning of the test was 1,000 mg / l. The PCB concentration of the slurry oxidized as described above was reduced to 0.94 mg / kg in terms of dry dry matter. Furthermore, when the dissolution test was carried out and the PCB concentration in the eluate was measured, no PCB was detected. As a result of comparing the present comparative example with the examples, it was confirmed that the method according to the present invention can be processed to the same degree as the conventional method using a catalyst, although no catalyst is used.

<Comparative example 2>
The contaminated soil was purified in the same manner as in Example 1 except that the temperature in the container during the reaction was 90 ° C. As a result, the PCB concentration after the treatment was 17.63 mg / kg in terms of dry dry matter, and the purification was not sufficiently performed.

<Comparative Example 3>
The contaminated soil was purified in the same manner as in Example 1 except that the temperature in the container during the reaction was 280 to 300 ° C. As a result, the PCB concentration after the treatment was 9.82 mg / kg in terms of dry dry matter, and the purification was not sufficiently performed.

  According to the present invention as described above, there is provided a method for purifying contaminated soil, which can efficiently and economically remove contaminants such as PCB from soil contaminated with PCB or the like and make them harmless.

It is a figure which shows the relationship between the temperature rise of water, and the change of a dielectric constant. It is a typical system diagram for demonstrating the soil purification processing method of this invention. It is the schematic of the apparatus in the case of applying the soil purification processing method concerning this invention to an actual soil treatment.

Explanation of symbols

1: Reaction tank 1a: Reaction vessel 2: Heater 3: Oxidizing agent 4: Slurry tank 5: Line mixer 6: Stirrer S: Soil (slurry)

Claims (3)

A method for purifying contaminated soil for removing at least one of PCB and dioxins from soil by oxidative decomposition, making it harmless and reusable, adding water to the contaminated soil and mixing it into a slurry, After adjusting the pH of the slurry to 4 or less and adding hydrogen peroxide , the slurry is filled in a pressure resistant container, the temperature in the container is 150 to 200 ° C., and the pressure in the container is 0.5 to 10 MPa. in the conditions, at least one of the PCB and dioxins are dissolved in water, without the use of a catalyst, oxidative decomposition by hydroxyl radicals generated from hydrogen peroxide (oxidative decomposition using an oxygen-containing gas as the oxidant A soil refining treatment method characterized by comprising:   The soil purification method according to claim 1, wherein a ratio of the soil to the water in the slurry is, by weight, soil: water = 1: 10 to 1: 2. A device for purifying contaminated soil, which can be made reusable by oxidizing and removing at least one of PCB and dioxins from the soil, adding water to the contaminated soil and mixing it into a slurry, and the slurry A slurry tank for adjusting the pH of the slurry to 4 or less, at least one line mixer for mixing the slurry with hydrogen peroxide, and a plurality of pressure-resistant reaction vessels filled with the slurry sent from the line mixer The container is prepared by dissolving at least one of the above PCBs and dioxins in water under conditions of a container internal temperature of 150 to 200 ° C. and a container internal pressure of 0.5 to 10 MPa in a state where the slurry is filled. , without using a catalyst, to oxidative decomposition using an oxygen-containing gas as the oxidizing decomposition (oxidizing agent with a hydroxy radicals generated from hydrogen peroxide A soil refining treatment apparatus characterized in that the above-described treatment is performed sequentially in multiple stages in a plurality of reaction vessels.

Images