Method for purifying contaminated soil containing lead

Abstract

To provide a method for purifying contaminated soil containing lead in which lead can be efficiently extracted and recovered from contaminated soil.SOLUTION: A method for purifying contaminated soil containing lead includes: mixing contaminated soil containing lead, amino polycarboxylic acid type chelator and water to form suspension; adjusting pH of the suspension within a range of 6 to 11; stirring the pH-adjusted suspension and then separating the suspension into a solid content including soil and liquid including chelated lead; adding a compound having dithiocarbamic acid group to the liquid and stirring it, and then precipitating as a solid matter including lead; and recovering a solid matter containing the lead.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] An embodiment of the present invention relates to a method for remediating contaminated soil containing lead, and more particularly to a remediation method including extracting lead from contaminated soil containing lead and recovering the lead as a solid material containing lead. [Background technology]

[0002] Soil contamination by heavy metals has long been a problem. For example, soil excavated during tunnel or dam construction often contains heavy metals such as lead resulting from artificial or natural contamination. Heavy metals resulting from natural contamination include arsenic, mercury, cadmium, lead, and hexavalent chromium. In some areas, excavated soil contains lead in excess of the standard value of 150 mg / kg set by the Soil Contamination Countermeasures Act. Furthermore, lead is the most commonly identified area designated as a specific hazardous substance. For these reasons, a method for purifying contaminated soil containing lead is desired.

[0003] The insolubilization method and the washing method are known as typical methods for purifying contaminated soil. The insolubilization method is a method in which contaminated soil containing contaminants such as heavy metals is mixed with a chemical to insolubilize the contaminants such as heavy metals and improve the properties of the contaminated soil. However, since the insolubilized contaminants remain in the soil, there is concern that the contamination problem will continue unless the insolubilization is sufficient. On the other hand, the washing method is a method in which contaminated soil containing heavy metals and other contaminants is washed with water or chemicals to dissolve and remove the contaminants. The washing method is advantageous in that it allows for large-scale, inexpensive treatment, and produces purified soil from which contaminants can be removed and which can be reused. However, strong acids such as hydrochloric acid and nitric acid, which are typically used as chemicals, can significantly change the inherent chemical, physical, and biological properties of the soil. Furthermore, the washing method requires the use of large amounts of water for washing.

[0004] In response to this, for example, Patent Document 1 discloses a method for purifying heavy metal-containing field soil, which is characterized in that the soil is purified by washing the heavy metal-containing field soil with an aqueous chemical solution or water in a washing tank and extracting the heavy metals, and that after adding a polymer flocculant to the washing solution, the soil slurry is dehydrated using a vent press. In the examples of Patent Document 1, it is made clear that cadmium can be efficiently removed from field soil by adding an aqueous ferric chloride solution to the cadmium-containing field soil, stirring it, and then adding a polymer flocculant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2010-201385 A Summary of the Invention [Problem to be solved by the invention]

[0006] On the other hand, Patent Document 1 describes that wastewater containing heavy metals obtained after purification of field soil can be treated with alkali and then neutralized before being discharged, but no specific study has been made on the treatment of the wastewater. Normally, in order to treat contaminated soil containing heavy metals by a washing method, not only is a large-scale device and a large amount of water required, but a large amount of sludge is also generated. Therefore, a method is desired that can efficiently remove heavy metals from contaminated soil and efficiently recover the heavy metals from the wastewater containing the removed heavy metals. In particular, from the viewpoint of reducing the volume of sludge, a simple method is desired that can recover heavy metals as small a solid as possible. Therefore, in view of the above-mentioned circumstances, an embodiment of the present invention provides a method for remediating contaminated soil containing lead, the method including simply and efficiently extracting and recovering lead from the contaminated soil containing lead. [Means for solving the problem]

[0007] The present inventors have conducted extensive research into methods for purifying contaminated soil containing lead by washing, and have found that lead can be efficiently extracted by using an aminopolycarboxylic acid-type chelating agent when washing contaminated soil, and that lead can be efficiently recovered as an insoluble solid by adding a specific chelating flocculant to the extracted lead-containing liquid, thereby completing the present invention. That is, the present invention relates to the embodiments described below. However, the present invention is not limited to the embodiments described below, and includes various embodiments.

[0008] <1> 1. A method for remediating contaminated soil containing lead, comprising: mixing contaminated soil containing lead, an aminopolycarboxylic acid type chelating agent, and water to form a suspension; adjusting the pH of the suspension to a range of 6 to 11; agitating the pH-adjusted suspension and then separating it into a solid fraction containing soil and a liquid fraction containing chelated lead; adding a compound having a dithiocarbamic acid group to the liquid, mixing the mixture, and then allowing a solid containing lead to settle; and recovering said lead-containing solids. A method for purifying contaminated soil, comprising:

[0009] <2> The aminopolycarboxylic acid type chelating agent includes at least one selected from the group consisting of ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and triethylenetetraminehexaacetic acid. <1> 2. A method for purifying contaminated soil according to claim 1.

[0010] <3> The aminopolycarboxylic acid type chelating agent is used in a molar ratio of 10 to 150 times based on the amount of lead contained in the contaminated soil. <1> or <2> 2. A method for purifying contaminated soil according to claim 1.

[0011] <4> The suspension is stirred under conditions in which the temperature of the suspension is 20°C to 50°C. <1> ~ <3> 2. A method for purifying contaminated soil according to claim 1 .

[0012] <5> The compound having a dithiocarbamic acid group is a dialkyldithiocarbamic acid or a metal salt thereof. <1> ~ <4> 2. A method for purifying contaminated soil according to claim 1 .

[0013] <6> The liquid fraction obtained after recovering the lead-containing solids is reused to form the suspension. <1> ~ <5> 2. A method for purifying contaminated soil according to claim 1 . Effect of the Invention

[0014] According to an embodiment of the present invention, it is possible to provide a method for remediating contaminated soil, which can efficiently extract and recover lead from contaminated soil containing lead. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a flow diagram illustrating a method for remediating contaminated soil according to an embodiment of the present invention. [Diagram 2] 1 is a graph summarizing the results of the lead removal rate by each chelating agent examined in the examples. [Diagram 3] 1 is a graph summarizing the relationship between the DDTC / [Pb-EDTA]2-stoichiometric ratio and the transfer rate of lead examined in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the embodiments described below and includes various embodiments.

[0017] One embodiment of the present invention is a method for purifying contaminated soil containing lead by a washing method, which includes a step of extracting lead from the contaminated soil and a step of recovering the lead in the liquid (aqueous phase) as a solid. Figure 1 is a flow diagram explaining the method for purifying contaminated soil according to the present invention. Each step will be explained in more detail below with reference to Figure 1. (Lead extraction process) In the purification method of the above embodiment, in the step of extracting lead, first, contaminated soil containing lead (Pb), an aminopolycarboxylic acid type chelating agent, and water are mixed to form a suspension. The mixing for forming the suspension can be performed by a method such as shaking or stirring that is commonly used in the art. In the above mixing, the aminopolycarboxylic acid type chelating agent and water may be added to the contaminated soil simultaneously or separately. In one embodiment, a chelating agent solution containing an aminopolycarboxylic acid type chelating agent and water may be prepared in advance and used.

[0018] In the above method, the contaminated soil to be purified is at least lead-containing soil, and may be soil containing 150 mg / kg or more of lead, which is the standard value set by the Soil Contamination Countermeasures Act. As the purification method, a method capable of removing lead from contaminated soil containing lead such that the lead content in the purified soil is below the standard value is preferable. According to the method for purifying contaminated soil of the present embodiment, lead can be efficiently extracted and efficiently recovered from contaminated soil containing lead, so that purified soil in which the lead content in the soil is below the standard value can be easily obtained. The contaminated soil contains silica, which is the main component of the soil, and heavy metals such as lead, and may further contain other metals such as iron and copper. In the contaminated soil, the heavy metals and metals may exist in various forms, and these may be in the form of, for example, an element, an ion, or a compound such as an oxide.

[0019] The particle size of the soil particles of the contaminated soil to be purified is not particularly limited, but may be approximately 2 mm or less. In one embodiment, it is preferable to perform pretreatment by air-drying, crushing, and sieving the excavated contaminated soil so that the soil particles have a particle size of approximately 2 mm or less. When water is added to the contaminated soil and wet classification is performed, there is a tendency for more lead to be contained in the small particle size fraction, for example, less than 75 μm, than in the large particle size fraction. This is thought to be because the addition of water to the soil causes water molecules to enter between the soil particles, weakening the bonds between the particles and increasing the amount of soil particles with a particle size of less than 75 μm, and further, lead does not exist as a compound but is locally adsorbed on the surface of the soil particles, or is adsorbed on the surface of metal oxides such as iron oxide and magnesium oxide and other organic matter.

[0020] Various compounds are known as chelating agents for heavy metals, but this embodiment is characterized by using an aminopolycarboxylic acid type chelating agent. The aminopolycarboxylic acid type chelating agent is likely to undergo a chelation reaction to form a complex with lead. Specific examples of aminopolycarboxylic acid type chelating agents include ethylenediaminetetraacetic acid (EDTA), trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA), diethylenetriaminepentaacetic acid (DTPA), and triethylenetetraminehexaacetic acid (TTHA). These may be used alone or in combination of two or more. As is clear from the examples described below, TTHA is the most excellent of the above chelating agents in terms of complex formation ability (lead extraction rate). However, EDTA can be used more preferably in consideration of the balance with cost.

[0021] From the viewpoint of increasing the extraction rate of lead by the aminopolycarboxylic acid type chelating agent, it is preferable to adjust the pH of the suspension to a range of 6 to 11. In order to adjust the pH of the suspension, an acidic compound or a basic compound can be used as necessary. Specific examples of compounds that can be used include sodium carbonate, sodium bicarbonate, tetrabutylammonium hydrate, iron(III) chloride hexahydrate, and sodium hydroxide. These compounds may be added when the contaminated soil, the aminopolycarboxylic acid type chelating agent, and water are mixed, or when the aminopolycarboxylic acid type chelating agent solution is prepared.

[0022] By adjusting the pH of the suspension to 6 or more, it is possible to suppress changes in the inherent properties of the soil caused by the suspension becoming acidic. On the other hand, it is generally expected that increasing the pH of the suspension will improve the extraction rate of lead from the viewpoint of the stability constant of a complex. However, the present inventors have confirmed that when the suspension becomes alkaline, salts such as hydroxide salts are easily formed, and the extraction rate of lead tends to decrease. For this reason, it is preferable to adjust the pH of the suspension to 11 or less. The pH of the suspension may be more preferably in the range of 6 to 9, and even more preferably in the range of 6 to 8.

[0023] The amount of the aminopolycarboxylic acid type chelating agent used is not particularly limited and can be adjusted appropriately. For example, the amount of the aminopolycarboxylic acid type chelating agent (chelating agent) used may be 10 to 150 times the amount of lead (Pb) contained in the contaminated soil in terms of the molar ratio of chelating agent / Pb. When the molar ratio of chelating agent / Pb is 10 or more, it is easy to obtain an excellent effect in Pb extraction by the chelating agent. On the other hand, even if the molar ratio of chelating agent / Pb exceeds 150, there is no significant change in the Pb extraction rate. Therefore, from the viewpoint of cost, the molar ratio may be 150 or less. The molar ratio may be more preferably 30 to 120, and further preferably 50 to 100.

[0024] The mixing of the contaminated soil, the aminopolycarboxylic acid type chelating agent, and the water is preferably carried out under temperature conditions where the temperature of the suspension is 20°C to 50°C. When the temperature of the suspension is adjusted to 20°C or higher, more preferably 25°C or higher, the Pb extraction rate tends to increase significantly. On the other hand, when the temperature of the suspension exceeds 30°C, the increase in the Pb extraction rate tends to become slower, and this tendency becomes even more pronounced when the temperature of the suspension exceeds 50°C. From this perspective, when the temperature of the suspension is adjusted to about 25°C to 30°C, it tends to be easier to efficiently extract Pb. The suspension is preferably stirred for a certain period of time after being adjusted to the above temperature range, the above chelating agent / Pb molar ratio, and the above pH range. The stirring time is not particularly limited, but may be, for example, 30 minutes to 10 hours, and preferably 1 to 6 hours.

[0025] In the purification method according to the present embodiment, the contaminated soil containing lead is mixed with an aminopolycarboxylic acid type chelating agent and water, thereby weakening the bonds between the soil particles, and the lead eluted from the soil particles is chelated by the aminopolycarboxylic acid type chelating agent to form a complex. The chelated lead moves into a liquid containing water, and can be easily separated from the solid content containing the soil particles by solid-liquid separation. Since the chelated lead can be separated (extracted) as a liquid, the lead content in the soil can be easily reduced by solid-liquid separation. The solid-liquid separation can be carried out according to a known method such as filtration separation, centrifugation, sedimentation, etc. As shown in Figure 1, the solid content (1) recovered after the solid-liquid separation can be dried and, if necessary, subjected to other treatments, and can be reused as purified soil.

[0026] (Lead recovery process) As shown in FIG. 1, in the purification method of the present embodiment, a compound having a dithiocarbamic acid group is added to a liquid fraction (1) obtained after solid-liquid separation. The liquid fraction (1) recovered after solid-liquid separation contains water used in forming the suspension and chelated lead (lead complex). By adding a compound having a dithiocarbamic acid group to the liquid fraction (1) and mixing it, the lead complex is immobilized and precipitated. The temperature during the above mixing is not particularly limited, and the mixture can be performed at room temperature, but may be heated as necessary.

[0027] As a method for treating wastewater containing heavy metals, for example, Patent Document 1 discloses a method of treating wastewater by adding an alkali to the wastewater. However, lead complexes such as lead-EDTA complexes formed using aminopolycarboxylic acid type chelating agents are stable. Therefore, even if a method of treating a liquid fraction containing a lead complex by adding an alkali is applied, it is presumed that it is difficult to efficiently recover the lead complex in the liquid fraction. Specifically, even if an alkali is added to a liquid fraction containing a lead complex, it is considered that lead hydroxides are difficult to precipitate and lead is likely to remain in the liquid fraction. In contrast, the purification method of the present embodiment uses a compound having a dithiocarbamic acid group for the lead complex, which is advantageous in precipitating the lead complex. It is considered that the lead-containing precipitate formed by using a compound having a dithiocarbamic acid group can reduce the amount of sludge generated compared to the precipitation of hydroxides.

[0028] The progress of the reaction between the lead complex and the compound having a dithiocarbamic acid group can be confirmed by the formation of a precipitate, and the precipitate containing lead can be recovered by solid-liquid separation of the solid content (2) containing the precipitate and the liquid content (2) containing water. The solid-liquid separation can be carried out according to a known method such as filtration separation, centrifugation, or sedimentation separation. The precipitate recovered from the solid content (2) is an insoluble solid material containing lead, and after drying, it can be transported directly to a heavy metal treatment facility or the like for disposal.

[0029] The compound having a dithiocarbamic acid group may be a compound known as a chelating agent that combines with lead to form an insoluble solid. Specific examples include dithiocarbamic acid or its salt, dialkyldithiocarbamic acid or its salt, cycloalkyldithiocarbamic acid or its salt, piperazinedithiocarbamic acid or its salt, tetraethylenepentaminedithiocarbamic acid or its salt, and dithiocarbamic acid of polyamine or its salt. Among them, dialkyldithiocarbamic acid or its salt can be preferably used. Although not particularly limited, the carbon number of the dialkyl in the dialkyldithiocarbamic acid may be 1 to 6. For example, diethyldithiocarbamic acid, sodium diethyldithiocarbamate, potassium diethyldithiocarbamate, etc. can be easily obtained as a commercially available product.

[0030] The amount of the compound having a dithiocarbamic acid group used is not particularly limited as long as it is sufficient to precipitate the lead complex in the liquid. In general, the stoichiometric ratio (molar ratio) of the compound having a dithiocarbamic acid group based on the lead complex in the liquid is preferably 1 to 50, more preferably 10 to 50, and even more preferably 20 to 50. When a compound having a dithiocarbamic acid group, such as diethylcarbamate (DDTC) or its salt, is added to and mixed with an aqueous solution containing a lead complex consisting of EDTA and lead, the DDTC / [Pb-EDTA] 2- The weight of the precipitate increases with an increase in the ratio of DDTC / [Pb-EDTA]. 2- When the volume ratio is 50, the actual weight of the precipitate is a very small value of 4.3 mg. Therefore, the addition of a compound having a dithiocarbamic acid group such as DDTC makes it possible to significantly reduce the volume of the sludge.

[0031] The mixing (stirring or shaking) after the addition of the compound having a dithiocarbamic acid group is not particularly limited. In one embodiment, the mixing is preferably performed under conditions where the temperature of the liquid content is 10 to 60° C. The temperature of the liquid content may be more preferably 20 to 50° C., and further preferably 20 to 40° C. When the temperature of the liquid content is adjusted to the above range, the precipitation of solids including lead tends to be promoted. The mixing can be carried out according to a method known in the art. The mixing time is not limited and can be adjusted as appropriate. For example, the temperature of the liquid is 25° C., the DDTC / [Pb-EDTA] 2- When the mixing ratio is adjusted to 50, the mixing time may be 0.2 to 4 hours, and more preferably 1 to 2 hours.

[0032] In the concentration (recovery) of lead (Pb) using compounds having a dithiocarbamic acid group such as DDTC, the reaction proceeds, for example, as shown in the following formula (I), and Pb forms a complex with DDTC and is concentrated as an insoluble precipitate (solid). [ka] On the other hand, it is possible that the bond between Pb and EDTA is broken by the formation of a complex between DDTC and Pb, leaving EDTA in the aqueous phase. Therefore, by performing solid-liquid separation, the liquid after Pb concentration (Pb recovery) can be reused as an EDTA solution to purify contaminated soil. EXAMPLES

[0033] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples and includes various embodiments.

[0034] <1> Soil sample preparation and analysis Contaminated soil containing lead was collected after excavation and removal from a former factory site. The contaminated soil was spread out in a shallow container, dried out of direct sunlight, and crushed using a mortar and pestle. The soil was then sieved to obtain soil samples with soil particle sizes ranging from 75 μm to 2 mm. 30 mL of 1 mol / L HCl was added to 0.9 g of the soil sample prepared as described above, and the suspension was shaken at 15 rpm and 25°C for 2 hours, and the resulting suspension was left to stand for 10 minutes. It was then filtered through a 0.45 μm membrane filter, and the metal ion concentration of the filtrate was measured by ICP-MS. The metal ion concentration obtained by the measurement was converted into the weight of the soil dried at 105°C for 4 hours to obtain the metal content in the soil. In addition, the ratio of the mass of water to the mass of soil particles among the soil particles, water, and air that constitute the soil was analyzed for the soil sample to obtain the water content in the soil sample. Furthermore, distilled water was added to the soil sample, and the pH was measured by the glass electrode method. These results are shown in Table 1.

[0035] [Table 1]

[0036] As shown in Table 1, the soil sample contained Pb in an amount far exceeding the standard value of 150 mg / kg set by the Soil Contamination Countermeasures Act. It also contained metals such as Fe, Cu, and Zn, with the Fe content being particularly high.

[0037] <2> Study on extraction of Pb from soil samples <2-1> Study of extraction effect by chelating agent 1 (Preparation of Chelating Agent Solution) First, a chelating agent solution was prepared as follows. EDTA solution: Weigh out 1.0437 g of ethylenediaminetetraacetic acid (molecular weight 292.24), add distilled water, and dissolve with NaOH while stirring. The volume is adjusted to 100 mL to obtain a solution of 3.57 × 10 -2 mol / L EDTA solution was prepared. This was used as the mother liquid. -3 A mol / L EDTA solution was prepared. Citric acid solution: Weigh out 0.6862 g of citric acid (molecular weight 192.12) and adjust the volume to 100 mL with distilled water to obtain a solution of 3.57 × 10 -2A citric acid solution of 3.57 × 10 mol / L was prepared. -3 A citric acid solution of 100 mol / L was prepared. Oxalic acid solution: Weigh out 0.4503 g of oxalic acid dihydrate (molecular weight 126.07) and adjust the volume to 100 mL with distilled water to obtain a solution of 3.57 × 10 -2 A citric acid solution of 3.57 × 10 mol / L was prepared. -3 A mol / L oxalic acid solution was prepared.

[0038] (Extraction of Pb using a chelating agent solution) Next, 20 mL of each of the EDTA solution, citric acid solution, and oxalic acid solution prepared as above was added as a chelating agent solution to 2 g of the soil sample (contaminated soil) prepared previously, and the suspension was obtained by shaking at 150 rpm for a predetermined time. The temperature of the suspension was adjusted to 20°C, and the molar ratio of chelating agent / Pb was adjusted to 10 and 100. After leaving the suspension to stand for 10 minutes, it was filtered through a 0.45 μm membrane filter to recover the filtrate. Next, the concentration of metal ions (Pb) contained in the filtrate was measured by ICP, and the extraction rate E [%] was calculated by the following (Equation 1). In addition, as a comparative control, the filtrate obtained by filtering the suspension formed without using the chelating agent solution was measured according to the same method as above, and the extraction rate E [%] was calculated. These results are shown in Table 2.

[0039]

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[0040] [Table 2] (1) The molar ratio of chelating agent / Pb is 10 (2) The molar ratio of chelating agent / Pb is 100

[0041] As is clear from the results shown in Table 2, when distilled water was used, the Pb extraction rate was very small. Therefore, it is considered difficult to extract Pb from soil using distilled water alone. In contrast, when EDTA was used as the chelating agent, the Pb extraction rate increased with increasing shaking time and chelating agent / Pb molar ratio. When the chelating agent / Pb molar ratio was 100 and the shaking time was 6 hours, an extraction rate of 93.8% was achieved. On the other hand, when citric acid was used, the effect of shaking time was small, and the Pb extraction rate increased with an increase in the molar ratio of chelating agent / Pb. However, the Pb extraction rate was significantly lower than when EDTA was used. When oxalic acid was used, the Pb extraction rate was significantly lower under all conditions. From the above, it can be seen that when EDTA is used as a chelating agent, Pb can be efficiently extracted (removed) from soil.

[0042] <2-2> Study on the extraction effect of chelating agents 2 (Preparation of Chelating Agent Solution) First, an aminopolycarboxylic acid type chelating agent solution was prepared as follows. EDTA solution: 52.1857 g of ethylenediaminetetraacetic acid (molecular weight 292.24) was weighed out, and distilled water was added. The mixture was dissolved with NaOH while stirring, and the volume was adjusted to 1 L to prepare a 0.179 mol / L EDTA solution. This was used as the mother liquid, and 1.79 × 10 -2 mol / L, 8.93×10 -3 mol / L, 1.79×10 -3 EDTA solutions of various mol / L concentrations were prepared. CyDTA solution: Weigh out 0.6185 g of trans-1,2-cyclohexanediaminetetraacetic acid monohydrate (molecular weight 346.34), add distilled water, and dissolve with NaOH while stirring. The volume is adjusted to 100 mL to obtain a solution of 1.79 × 10 -2 A CyDTA solution of 8.93 × 10 mol / L was prepared. -3 mol / L, 1.79×10 -3 CyDTA solutions of various mol / L concentrations were prepared. DTPA solution: Weigh out 0.7024 g of diethylenetriaminepentaacetic acid (molecular weight 393.35), add distilled water, and dissolve with NaOH while stirring. The volume is adjusted to 100 mL to obtain a solution of 1.79 × 10 -2 A DTPA solution of 8.93 × 10 mol / L was prepared. -3 mol / L, 1.79×10 -3 DTPA solutions of various mol / L concentrations were prepared. TTHA solution: Weigh out 0.8829 g of triethylenetetraminehexaacetic acid (molecular weight 494.45), add distilled water, dissolve with NaOH while stirring, and adjust the volume to 100 mL to obtain a solution of 1.79 × 10 -2 A TTHA solution of 8.93 × 10 mol / L was prepared. -3 mol / L, 1.79×10 -3 TTHA solutions of various mol / L concentrations were prepared.

[0043] (Extraction of Pb using a chelating agent solution) Next, 20 mL of each of the chelating agent solutions prepared as above was added to 2 g of the soil sample (contaminated soil) previously prepared as a chelating agent solution, and the suspension was obtained by shaking at 150 rpm for 6 hours. The temperature of each suspension was adjusted to 20 to 50°C, the pH to 5.6 to 9.9, and the molar ratio of chelating agent / Pb to 10, 50, and 100. Next, the suspension was left to stand for 10 minutes and then filtered through a 0.45 membrane filter. The metal ion concentration of the filtrate was measured by IPC-MS, and the extraction rate E [%] was calculated by the above-mentioned formula (1). A graph summarizing these results is shown in Figure 3.

[0044] As shown in Figure 3, it was confirmed that all the chelating agents used were able to efficiently remove Pb from soil. This is believed to be because the chelating agents form Pb complexes that dissolve into the solution. Among the chelating agents used, TTHA had the highest Pb removal rate, but EDTA can be used preferably from a cost perspective.

[0045] <2-3>Effect of other elements on Pb removal rate In the previous study, it was found that Pb can be efficiently removed (extracted) from soil using aminopolycarboxylic acid-type chelating agents such as EDTA. However, since other metals contained in the soil may form complexes with EDTA and hinder the removal of Pb, the effects of other metals were examined. In the same manner as described above, the EDTA solution was added to the soil sample and the suspension was obtained by shaking at 150 rpm for 6 hours. The temperature of the suspension was adjusted to 20°C, the pH to 7.2-7.5, and the EDTA / Pb molar ratio to 10, 50, and 100. The suspension was then left to stand for 10 minutes and filtered through a 0.45 membrane filter. The metal ion concentration of the filtrate was measured by IPC-MS, and the amount of removal was calculated from the value. Furthermore, the extraction rate (removal rate) E [%] was calculated using the formula (I) described above. These results are shown in Table 3.

[0046] [Table 3]

[0047] As shown in Table 3, for all metal elements, the amount removed increased as the EDTA / Pb molar ratio increased. Despite the large amount of Fe contained in the soil, when the EDTA / Pb molar ratio was 10, the amount of Pb removed was the greatest compared to the other metals. Since there was almost no difference in the stability constants of the complexes of each metal element, it is believed that Pb does not exist as a compound, but is adsorbed to the soil and is therefore more easily eluted than other metal elements.

[0048] <Example 1> (1) Preparation of solutions The liquid fraction recovered after solid-liquid separation of the suspension obtained by adding EDTA solution to a soil sample and mixing it contains chelated lead. As an alternative solution for this liquid, [Pb-EDTA] 2- A DDTC solution to be added to the liquid was prepared as follows. Pb-EDTA 2-Solution: Weigh out 0.2707 g of disodium ethylenediaminetetraacetate (II) hydrate (molecular weight 541.39) and make the solution 500 mL with distilled water to obtain a 1.0 mmol / L [Pb-EDTA] solution. 2- A solution was obtained. DDTC solution: 2.2531g of sodium diethyldithiocarbamate (molecular weight 225.31) was weighed out and the volume was adjusted to 250mL with distilled water to obtain a 40mmol / L DDTC solution. Using this as the mother liquor, DDTC solutions with concentrations of 16mmol / L, 8.0mmol / L, 4.0mmol / L, and 0.8mmol / L were prepared. (2) Concentration of Pb by DDTC [Pb-EDTA] prepared as above 2- 50 mL of DDTC solution was added to 20 mL of the solution, and the mixture was stirred at 25°C and 300 ppm for 1 hour, and then centrifuged at 10,000 rpm for 20 minutes. The precipitate was then collected by filtration through a 0.1 μm membrane filter. The collected precipitate was dried and weighed, and the weight reduction rate was calculated using the following formula (2). The Pb content in the aqueous phase was also calculated. 2+ The concentration was measured by ICP, and the transfer rate T [%] was calculated by the following formula 3.

[0049]

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number

[0050] In the above formula (2), Wwo [mL] is the initial weight of the aqueous phase, Ww1 [g] is the weight of the precipitate, and in the above formula (3), Cwo [mol / L] is the initial metal ion concentration in the aqueous phase, and Cw1 [mol / L] is the metal ion concentration in the aqueous phase after the experiment.

[0051] <Comparative Example 1> (1) Preparation of solutions The precipitate was collected in the same manner as in Example 1, except that an ion associate was used instead of the DDTC solution described in Example 1. The ion associate contained DBS as an organic anion. - , HTA as organic cation + Combination with (DBS - / HTA + = 10), and each solution was prepared as follows. (Ion associates) DBS - Solution: Weigh out 3.4848 g of sodium dodecylbenzenesulfonate (molecular weight 348.48, Kanto Chemical Co., Ltd., purity >90.0%) and adjust the volume to 50 mL with distilled water to obtain 0.2 mol / L DBS. - A solution was prepared. HTA + Solution: Weigh out 1.6 g of hexadecyltrimethylammonium (molecular weight 320, Tokyo Chemical Industry Co., Ltd., purity >95.0%) and adjust the volume to 50 mL with distilled water to obtain 0.1 mol / L HTA. + A solution was prepared. (2) Concentration of Pb by ion association [Pb-EDTA] prepared in the same manner as in Example 1 2- 0.2 mol / L DBS per 10 mL of solution - 5 mL of solution, and 0.1 mol / L HTA + 0.5 mL of the solution was added. The mixture was then centrifuged at 25°C and 3000 rpm for 20 minutes to separate the aqueous phase from the ion association phase. The precipitate was then collected by filtration through a 0.1 μm membrane filter. The collected precipitate was dried and weighed, and the weight reduction rate was calculated using the above formula (2). The Pb content in the aqueous phase was also calculated. 2+ The concentration was measured by ICP, and the transfer rate T [%] was calculated by the above (Equation 3).

[0052] The precipitate weight and weight loss rate at each stoichiometric ratio for Example 1 are shown in Table 4. In addition, a graph summarizing the Pb transfer rate at each stoichiometric ratio is shown in Figure 3. [Table 4]

[0053] As can be seen from Table 4, DDTC / [Pb-EDTA] 2- The weight of the precipitate increased with increasing stoichiometry. The weight of the precipitate was calculated as DDTC / [PbEDTA] 2- Even at a stoichiometric ratio of 50, the value is very small at 4.3 mg. Therefore, the addition of DDTC achieved a reduction in sludge volume. Similarly, as can be seen from Figure 3, the Pb transfer rate was also significantly reduced with the addition of DDTC / [Pb-EDTA]. 2- The stoichiometric ratio showed a tendency to increase with increasing DDTC / [Pb-EDTA]. 2- When the stoichiometric ratio was 50, the Pb transfer rate reached 90%. On the other hand, in Comparative Example 1, in which ion associates were used instead of DDTC, the extraction rate was 50.1% and the volume reduction rate was 1.21. Since the volume reduction rate is a low value, it is considered that sludge volume reduction is possible. However, since the Pb migration rate was 5.67%, which is a very low value, it was found that it is difficult to migrate Pb to the ion associate phase.

[0054] <Example 2> The extraction and recovery of Pb from soil samples was investigated as follows: Soil samples and EDTA solutions were prepared in the same manner as described above. First, 20 mL of EDTA solution with an EDTA / Pb molar ratio of 100 was added to 2 g of soil sample, and the mixture was shaken at 25°C, initial pH 7, 150 rpm for 6 hours, and then allowed to stand for 10 minutes. It was then filtered through a 0.45 μm membrane filter. To examine the effect of adsorbed water in the soil, a sample in which 20 mL of distilled water was added to wash the soil was compared with a sample to which no distilled water was added. The metal ion concentration of the filtrate was measured by ICP, and the extraction rate E [%] was calculated from the above (Equation 1). 10 mL of each filtrate was collected, 20 mL of DDTC solution adjusted to a DDTC / Pb stoichiometric ratio of 50 was added, and the mixture was stirred at 25°C and 300 rpm for 1 hour. After centrifuging at 10,000 rpm for 20 minutes, the mixture was filtered through a 0.10 μm membrane filter to obtain a precipitate. The weight of the precipitate after drying was measured, and the weight reduction rate WL [-%] was calculated from the above (Equation 2). In addition, the metal ion concentration of the filtrate obtained during the above filtration was measured by ICP, and the migration rate T [%] was calculated from the above (Equation 3). The results are shown in Table 5.

[0055] [Table 5]

[0056] The Pb extraction rate was almost the same whether or not distilled water was added. Therefore, it was found that the effect of adsorbed water was small, and a high Pb extraction rate could be achieved without washing the adsorbed water. In addition, [Pb-EDTA] was used as a simulant solution. 2- As with the case of using the solution, the weight reduction rate was small, and the Pb extracted into the aqueous phase could be concentrated.

[0057] Table 6 shows the amounts of Pb extracted and concentrated when distilled water was not added. [Table 6]

[0058] As shown in Table 6, the amount of Pb extracted was 327 mg / kg, and the amount of Pb remaining in the soil was 20.4 mg / kg. Therefore, the standard value of 150 mg / kg for the Pb content in the soil was met. In addition, the Pb concentration in the solid content (solid phase) was 303 mg / kg, which shows that most of the Pb in the soil can be recovered as a solid. From the above, it is found that lead can be efficiently eluted from soil particles by mixing soil particles containing lead, an aminopolycarboxylic acid type chelating agent, and water, and further, lead can be efficiently recovered as a solid material containing lead by adding a compound having a dithiocarbamic acid group such as DDTC to the liquid containing lead extracted from the soil particles and mixing it. Therefore, it is found that the present invention can realize a purification method that can easily and efficiently extract and recover lead from contaminated soil containing lead and purify the contaminated soil.

Claims

1. 1. A method for remediating contaminated soil containing lead, comprising: mixing contaminated soil containing lead, an aminopolycarboxylic acid type chelating agent, and water to form a suspension; adjusting the pH of the suspension to a range of 6 to 11; agitating the pH-adjusted suspension and then separating it into a solid fraction containing soil and a liquid fraction containing chelated lead; adding a compound having a dithiocarbamic acid group to the liquid, mixing the compound, and then precipitating the compound as a lead-containing solid; and recovering said lead-containing solids. A method for purifying contaminated soil, comprising:

2. The method for purifying contaminated soil according to claim 1, wherein the aminopolycarboxylic acid type chelating agent comprises at least one selected from the group consisting of ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and triethylenetetraminehexaacetic acid.

3. 3. The method for purifying contaminated soil according to claim 1, wherein the aminopolycarboxylic acid type chelating agent is used in a molar ratio of 10 to 150 times the lead contained in the contaminated soil.

4. 3. The method for remediating contaminated soil according to claim 1, wherein the suspension is stirred under conditions in which the temperature of the suspension is 20°C to 50°C.

5. 3. The method for remediating contaminated soil according to claim 1, wherein the compound having a dithiocarbamic acid group is a dialkyldithiocarbamic acid or a metal salt thereof.

6. 3. The method for purifying contaminated soil according to claim 1, wherein a liquid fraction obtained after recovering the lead-containing solid fraction is reused to form the suspension.

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