Insolubilization materials and insolubilization methods for contaminated soil

Abstract

To provide: an insolubilization material which is capable of preventing an increase in cost and sufficiently producing insolubilization effect for heavy metals, and which makes it possible to obtain insolubilized soil having sufficiently high solidification intensity; and an insolubilization method using the insolubilization material.SOLUTION: An insolubilization material for a contaminated soil in the present invention includes magnesium oxide powder containing sulfate, and ferrous chloride. A content rate of the sulfate in the magnesium oxide powder is 0.07-6.73 mass% in terms of SO3.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an insolubilizing material and an insolubilizing method for soil contaminated with heavy metals. [Background technology]

[0002] Excavation work for tunnels and the like generates soil containing naturally occurring heavy metals such as lead, arsenic, fluorine, and selenium. In recent years, insolubilization treatment, which can suppress the elution of pollutants at low cost and in a short time, has attracted attention as a method for treating soil contaminated by heavy metals. Patent documents 1 and 2 disclose an insolubilizer for heavy metals that utilizes the reducing action of ferrous chloride.

[0003] On the other hand, at construction sites, etc., soil after insolubilization treatment may be effectively used as improved soil, so it is desirable for the soil to have excellent improvement characteristics (solidification performance). Patent Document 3 discloses an insolubilizer containing light-burned magnesia partial hydrate, which further contains gypsum from the viewpoint of increasing the solidification strength. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-256593 A [Patent Document 2] Patent No. 6355946 [Patent Document 3] Patent No. 4481360 Summary of the Invention [Problem to be solved by the invention]

[0005] The insolubilizer described in Patent Document 1 does not have sufficient solidification performance for insolubilized soil, which results in a large amount of insolubilizer being added, resulting in high processing costs in some cases. The insolubilizers described in Patent Documents 2 and 3 have the risk of reducing the pH buffering capacity of the surrounding environment (groundwater, etc.) of the treated soil due to calcium ions derived from gypsum and calcium carbonate, and reducing the insolubilization effect of heavy metals.

[0006] The present invention has been made in consideration of the above problems, and aims to provide an insolubilizing material and an insolubilization method using the same, which can suppress increases in costs, fully exert the insolubilizing effect of heavy metals, and produce insolubilized soil with sufficiently high solidification strength. [Means for solving the problem]

[0007] Means for Solving the Problems The present inventors conducted extensive research to solve the above problems and discovered the following, which led to the completion of the present invention. (1) Magnesium oxide powder, which has traditionally been used as one of the raw materials for insolubilizing agents, contains sulfate as an impurity. Relatively low-quality magnesium oxide powder has the advantage of reducing raw material costs, but it contains a large amount of sulfate. (2) In an insolubilizing material containing magnesium oxide powder and ferrous chloride, sulfate contained as an impurity in the magnesium oxide powder has the effect of inhibiting the insolubilization of heavy metals, but on the other hand, it has the effect of increasing the strength expression of the insolubilizing material. (3) In an insolubilizing material containing magnesium oxide powder and ferrous chloride, by setting the amount of sulfate within a specific range, it is possible to achieve both the insolubilizing effect and strength development.

[0008] The insolubilizing agent for contaminated soil according to the present invention contains magnesium oxide powder containing sulfate and ferrous chloride, and the sulfate content of the magnesium oxide powder is 0.07 to 6.73 mass% calculated as SO3. This insolubilizing agent allows magnesium oxide powder containing a relatively large amount of sulfate (magnesium oxide powder with a relatively low purity) to be used as one of the raw materials, thereby suppressing an increase in costs. Furthermore, this insolubilizing agent contains the above-mentioned predetermined amount of sulfate, so that the insolubilizing effect of heavy metals is fully exerted and an insolubilized soil with sufficiently high solidification strength can be obtained. Note that the contaminated soil referred to here also includes rocks (excavated waste).

[0009] The magnesium oxide powder preferably has a magnesium oxide content of 80% by mass or more and a calcium oxide content of 3% by mass or less. By using magnesium oxide powder that satisfies these conditions, it is possible to sufficiently prevent a decrease in the pH buffering ability of the surrounding environment (groundwater, etc.) of the treated soil, and to more stably exert an excellent insolubilization effect of heavy metals.

[0010] The insolubilizing material for contaminated soil according to the present invention contains, for example, 20 to 80 mass % of magnesium oxide powder and 20 to 80 mass % of ferrous chloride, based on the total mass of the insolubilizing material.

[0011] The method for insolubilizing contaminated soil according to the present invention includes a step of mixing the insolubilizing agent for contaminated soil with soil. This insolubilizing method uses an insolubilizing agent made from magnesium oxide powder, which contains a relatively large amount of impurities (sulfates), as one of its raw materials, and therefore can suppress increases in costs. Furthermore, by using the insolubilizing agent, this insolubilizing method can obtain insolubilized soil that is fully effective in insolubilizing heavy metals and has sufficiently high solidification strength. Effect of the Invention

[0012] According to the present invention, an insolubilizing material and an insolubilization method are provided that can suppress increases in costs, fully exert the insolubilizing effect of heavy metals, and produce insolubilized soil with sufficiently high solidification strength. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail. However, the present invention is not limited to the following embodiment.

[0014] [Insolubilizer] The insolubilizing material according to the present embodiment contains magnesium oxide powder containing impurities and ferrous chloride, and the magnesium oxide powder contains 0.07 to 6.73 mass% of sulfate in terms of SO3. Note that "magnesium oxide" here refers to light-burned magnesium oxide obtained by firing magnesium hydroxide (Mg(OH)2) or magnesite ore (MgCO3) at a low temperature of 600 to 900°C.

[0015] Industrially produced magnesium oxide powder is obtained by calcining magnesium hydroxide (Mg(OH)2) produced by the reaction of seawater with lime or magnesite ore (MgCO3). Magnesium oxide powder obtained in this way may contain sulfates (e.g. MgSO4, CaSO4, and Na2SO4) derived from seawater or minerals as impurities.

[0016] In this embodiment, magnesium oxide powder containing a specific amount of sulfate is used. The content of sulfate in the magnesium oxide powder (based on the mass of the magnesium oxide powder) is 0.07 to 6.73 mass% in SO3 conversion, preferably 0.08 to 3.67 mass%, more preferably 0.09 to 2.60 mass%, and even more preferably 0.11 to 2.00 mass%, from the viewpoint of both the insolubilization effect and the strength development and the raw material cost. Specifically, when magnesium oxide powder having a sulfate content (SO3 conversion) of less than 0.07 mass% is used, the strength development is insufficient and the raw material cost increases. On the other hand, when magnesium oxide powder having a sulfate content (SO3 conversion) of more than 6.73 mass% is used, the insolubilization effect of heavy metals is insufficient. As for the magnesium oxide powder used in this embodiment, it is preferable that the content (SO3 conversion) of magnesium sulfate, particularly magnesium sulfate (MgSO4), is in the above range among sulfates.

[0017] The magnesium oxide powder used in this embodiment preferably has an MgO content (based on the mass of the magnesium oxide powder) of 80% by mass or more and a CaO content (based on the mass of the magnesium oxide powder) of 3% by mass or less. By using magnesium oxide powder that satisfies these conditions, it is possible to sufficiently suppress the decrease in the pH buffering ability of the surrounding environment (groundwater, etc.) of the treated soil, and the excellent insolubilization effect of heavy metals is more stably exhibited. The MgO content of the magnesium oxide powder is more preferably 85% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more. The CaO content of the magnesium oxide powder is more preferably 2.8% by mass or less, even more preferably 2.5% by mass or less, and particularly preferably 2.0% by mass or less.

[0018] The ferrous chloride used in the present embodiment may be commercially available and may be in either powder or liquid form, but it is preferable to use the powder form from the viewpoint of ease of handling. The ferrous chloride may be any of anhydrate, dihydrate, and tetrahydrate.

[0019] The insolubilizer of this embodiment can be obtained by mixing the magnesium oxide powder and the ferrous chloride. The magnesium oxide powder content of the insolubilizer (based on the amount of insolubilizer) is preferably 20 to 80 mass%, more preferably 40 to 70 mass%. The ferrous chloride content of the insolubilizer (based on the amount of insolubilizer) is preferably 20 to 80 mass%, more preferably 30 to 60 mass%.

[0020] The method for mixing the magnesium oxide powder and ferrous chloride is not particularly limited, and examples thereof include methods using equipment such as a ribbon mixer, a Nauta mixer, a drum blender, and a rocking mixer.

[0021] [Insolubilization method] The insolubilization method of this embodiment includes a step of mixing the insolubilizer with soil. The harmful substances in the soil can be insolubilized by the action and effect of the insolubilizer. In this embodiment, the harmful substances (heavy metals) contained in the soil to be treated are at least one substance selected from the group consisting of lead, hexavalent chromium, mercury, cadmium, arsenic, selenium, fluorine, and boron. The insolubilization method of this embodiment exerts an excellent insolubilization effect and strength development property on soil contaminated by heavy metals.

[0022] In the insolubilization method of this embodiment, the amount of insolubilizer to be added is selected depending on the soil quality and the degree of contamination of the target soil. 3 Adding 20 to 200 kg of the insolubilizer to the soil will provide sufficient insolubilization and solidification performance. The amount of the insolubilizer to be added is more preferably 20 to 150 kg / m 3 , more preferably 20 to 75 kg / m 3 The amount of insolubilizer added is 20 kg / m 3 If the amount of the insolubilizer is more than 200 kg / m, the insolubilizer can be mixed with the soil sufficiently, which is preferable. 3 If it is less than this, it is economical in terms of processing costs and is therefore preferable.

[0023] The amount of heavy metals eluted from insolubilized soil is required to meet the soil elution amount standard (for example, 0.01 mg / L or less for selenium) set forth in the Environment Agency Notification No. 18 Law, but taking into consideration the variation in the insolubilization effect due to the degree of mixing of the soil to be treated with the insolubilizing material, analytical errors, etc., it is desirable that the amount of selenium eluted, for example, be 0.007 mg / L or less. The insolubilization method according to this embodiment is particularly effective in insolubilizing the selenium contained in the soil to be treated, and can effectively reduce the amount of selenium eluted from the soil to be treated to 0.007 mg / L or less at low cost.

[0024] The cone index, which is an index of the improvement characteristics (solidification performance) of insoluble treated soil, is, for example, 1200 kN / m 2 It is desirable that the cone index is equal to or greater than the Japan Road Association's required cone index for dump trucks. In general, the relationship between the unconfined compressive strength (qu) and the cone index (qc) is qc=(5~10)qu(kN / m 2 ) (Solidification Method Research Group, Technical Documents, Chapter 4, Design), the unconfined compressive strength of insolubilized soil is 240kN / m 2 Since the insolubilizer of this embodiment has an excellent effect of enhancing the solidification performance of the insolubilized soil, the insolubilization method using this insolubilizer can increase the unconfined compressive strength of the insolubilized soil to 240 kN / m or more. 2 It can be increased to more than that. EXAMPLES

[0025] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0026] 1.Materials used [Original soil] Sandstone was used as the raw soil. The properties of the raw soil are shown in Table 1. The moisture content was measured in accordance with JIS A 1203 "Testing method for moisture content of soil." Grain size was calculated by washing the raw soil with water on a sieve with 75 μm openings to thoroughly wash away coarse particles, drying the entire amount of the sieve residue at 110°C for one day, and classifying the resulting dried soil using sieves with 2 mm, 425 μm, and 75 μm openings. pH was measured in accordance with JGS 0211-2009 "Testing method for pH of soil suspension."

[0027] [Table 1]

[0028] [Sample soil] A specified amount of an aqueous solution containing sodium selenate (Na2SeO4, manufactured by Wako Pure Chemical Industries, Ltd.) was added to the raw soil A shown in Table 1, and the mixture was mixed for 90 seconds using a soil mixer. The soil adhering to the container and paddle was then scraped off, and the mixture was mixed for another 90 seconds. The soil was scraped off again, the mixture was mixed for 90 seconds, and the sample soil was sealed in a polyethylene bag and cured for 8 days to obtain sample soil A (simulated contaminated soil). The properties of sample soil A (simulated contaminated soil) are shown in Table 2. The water content was measured in the same manner as for the raw soil. The wet density was determined by filling the sample soil into a formwork with a diameter of 5 cm and a height of 10 cm, and then calculating the mass of the filled sample soil and the volume of the formwork. The amount of selenium eluted was measured by preparing a test solution in accordance with the Environment Agency Notification No. 18, and the total selenium concentration of the test solution was measured in accordance with JIS K 0102:2016 "Industrial wastewater test method".

[0029] [Table 2]

[0030] A specified amount of distilled water was added to raw soil B shown in Table 1 and mixed using a soil mixer for 90 seconds, after which the soil adhering to the container and paddle was scraped off and mixed for another 90 seconds. The soil was scraped off again and mixed for 90 seconds, after which the sample soil was cured for one day in a sealed polyethylene bag to produce sample soil B. The properties of sample soil B are shown in Table 3. The water content was measured in the same manner as for the raw soil. The wet density was measured in the same manner as for sample soil A (simulated contaminated soil).

[0031] [Table 3]

[0032] [Magnesium oxide powder] As magnesium oxide powder, light-burned magnesium oxide powder (manufactured by Ube Material Industries, Ltd.) and anhydrous magnesium sulfate (MgSO4, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were used. The chemical composition of the light-burned magnesium oxide powder is shown in Table 4. The chemical composition of the anhydrous magnesium sulfate is shown in Table 5. A predetermined amount of anhydrous magnesium sulfate was added to this light-burned magnesium oxide powder to prepare magnesium oxide powders with different SO3 contents (simulated magnesium oxide powders containing impurities). The chemical composition of the prepared magnesium oxide powder is shown in Table 6. The ignition loss (ig.loss) of the light-burned magnesium oxide powder was obtained from the weight loss when the sample was ignited at 1000°C until it reached a constant weight. The SiO2 (including insoluble residue) and SO3 contents in the light-burned magnesium oxide powder were measured with reference to JIS R 9101:2018 "Methods for chemical analysis of gypsum". That is, the content of SiO2 (including insoluble residue) in the light-burned magnesium oxide powder was measured by dissolving the sample in hydrochloric acid and perchloric acid, heating to dehydrate SiO2 and make it insoluble, filtering the soluble part, igniting the SiO2 and insoluble residue on the filter paper, and weighing the mass. The content of SO3 in the light-burned magnesium oxide powder was measured by adding hydrochloric acid and water to the sample, boiling to dissolve the soluble residue, filtering with filter paper, adding barium chloride solution to the obtained filtrate and washings to precipitate barium sulfate, filtering the precipitate, igniting, and weighing the mass. The contents of Al2O3, Fe2O3, CaO, and MgO in the light-burned magnesium oxide powder were measured with reference to the test method for alkali metal elements (Na, K) in JIS K 8432:2017 "Magnesium oxide (reagent)". That is, after adding water and hydrochloric acid to the sample and heating to dissolve it, Fe2O3 was measured by absorptiometry, and Al2O3, CaO, and MgO were measured by ICP atomic emission spectrometry. The chemical composition of magnesium sulfate anhydrate was measured by alkali fusion-ICP atomic emission spectrometry. The chemical composition of magnesium oxide powder was calculated from the blending ratio of light-burned magnesium oxide powder and magnesium sulfate.

[0033] [Table 4]

[0034] [Table 5]

[0035] [Table 6]

[0036] [Ferrous chloride] Ferrous chloride tetrahydrate (FeCl2·4H2O, Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the ferrous chloride.

[0037] [Insolubilizer] The insolubilizer was prepared by mixing magnesium oxide powder and ferrous chloride in the prescribed ratio shown in Table 6. The mixing ratio of the insolubilizer is shown in Table 7.

[0038] [Table 7]

[0039] 2. Test Method [Insolubilization test] The various insolubilizing materials shown in Table 7 were applied to 1 m of sample soil A (simulated contaminated soil). 3 35.7 kg of selenium was added to the container and mixed for 90 seconds using a soil mixer, after which the soil adhering to the container and paddle was scraped off, and the soil was mixed for another 90 seconds to perform insolubilization treatment (Comparative Example 1, Examples 1 to 6, and Comparative Example 2). The insolubilized soil obtained was sealed in a polyethylene bag and cured at 20°C for one day, after which a test solution was prepared in accordance with Notification No. 18 of the Environment Agency. The total selenium concentration of the test solution was measured in accordance with JIS K 0102:2016 "Testing Methods for Industrial Wastewater".

[0040] [Solidification performance] The various insolubilizing materials shown in Table 7 were 3After mixing for 90 seconds using a soil mixer, the soil adhering to the container and paddle was scraped off, and the soil was mixed for another 90 seconds to perform insolubilization treatment (Comparative Example 1, Examples 1 to 6, and Comparative Example 2). The obtained insolubilized soil was filled into a formwork with a diameter of 5 cm and a height of 10 cm to prepare a cylindrical specimen, which was then sealed and cured at 20°C for one day, and its unconfined compressive strength was measured in accordance with JIS A 1216:2009 "Unconfined compression test for soil".

[0041] 3. Test Results The amount of selenium dissolved from the insolubilized soil and the unconfined compressive strength of the insolubilized soil are shown in Table 8. The amount of selenium dissolved was reduced to 0.007 mg / L or less, and the unconfined compressive strength was 240 kN / m. 2 Cases where the above conditions were met were rated as ○, and cases other than the above were rated as ×. Table 8 shows the measured values ​​of the amount of selenium eluted and the unconfined compressive strength of the untreated soil as Reference Example 1.

[0042] [Table 8]

[0043] As shown in Table 8, magnesium oxide powder containing 0.07 to 6.73 mass% of sulfate in SO3 equivalent and insolubilizer 2 to ferrous chloride are used. 7 It can be seen that (Examples 1 to 6) have a higher selenium insolubilizing effect than insolubilizer 8 (Comparative Example 2) containing magnesium oxide powder with a high sulfate content and ferrous chloride, and have a higher unconfined compressive strength than insolubilizer 1 (Comparative Example 1) containing magnesium oxide powder with a low sulfate content and ferrous chloride. As a result, the amount of selenium eluted was reduced to 0.007 mg / L or less, and the unconfined compressive strength was 240 kN / m 2 Met the above

[0044] The insolubilizer containing magnesium oxide powder not containing a specified amount of sulfate and ferrous chloride (Comparative Example 1) had insufficient solidification performance of the insolubilized soil. This resulted in an increase in the amount of insolubilizer added, which resulted in higher processing costs. In addition, insolubilizers containing light-burned magnesium partial hydrate and gypsum, and insolubilizers containing a magnesium-containing substance, a reducing substance such as ferrous chloride, and auxiliary materials such as gypsum powder and calcium carbonate-containing powder, the calcium ions derived from the gypsum and calcium carbonate may reduce the pH buffering capacity of the surrounding environment (groundwater, etc.) of the treated soil, and may also reduce the insolubilization effect of heavy metals including selenium. In contrast, the insolubilizers according to the above examples provide a solidification capacity of 30 to 40 kg / m 3 The amount of selenium eluted can be reduced to 0.007mg / L or less, and the unconfined compressive strength can be increased to 240kN / m 2 It can be increased even further.

Claims

1. An insolubilizing material for reducing the amount of heavy metal elution from contaminated soil containing at least one heavy metal selected from the group consisting of lead, hexavalent chromium, mercury, cadmium, arsenic, selenium, fluorine, and boron, comprising: Magnesium oxide powder containing sulfate; Ferrous chloride, Including, The sulfate content of the magnesium oxide powder is SO ３ An insolubilizing material for contaminated soil, which is equivalent to 1.40 to 5.41 mass%.

2. 2. The insolubilizing material for contaminated soil according to claim 1, wherein the magnesium oxide powder has an MgO content of 80 mass % or more and a CaO content of 3 mass % or less.

3. 3. The insolubilizing material for contaminated soil according to claim 1, comprising 20 to 80 mass% of the magnesium oxide powder and 20 to 80 mass% of the ferrous chloride, based on the total mass of the insolubilizing material.

4. A method for insolubilizing contaminated soil, comprising a step of mixing the insolubilizing material according to any one of claims 1 to 3 with soil.

Images