JP5887972B2 - Treatment method of contaminated soil - Google Patents

Description

  The present invention provides a method for treating arsenic-contaminated soil that exceeds the soil elution standard.

  In recent years, it has been reported that soil has been contaminated with harmful substances in sites such as factories, offices, and industrial waste treatment plants. For example, hexavalent chromium is used in plating plants, and fluorine and boron are used in glass processing plants, and there are cases where these harmful substances are detected at the sites of factories and offices. It is also known that soils contaminated with natural arsenic, fluorine, etc. also exist. Arsenic is relatively abundant in andesite and granite, and arsenic may be detected in soil and groundwater in areas where these rocks are distributed. Fluorine is contained in clay and silt deposits in the coastal area, and by including naturally derived fluorine in the soil, it may exceed the soil elution amount standard described in Schedule 3 of the Enforcement Regulations of the Soil Contamination Countermeasures Law is there.

  On the other hand, the treatment of contaminated soil in Japan is biased toward excavation and removal treatment, and the environmental load and economic load due to the distribution of a large amount of contaminated soil have become major issues. Based on this situation, the Soil Contamination Countermeasures Law has been revised and implemented to reduce human health risks and solve the Brownfield problem. Establishment of insolubilization technology is expected.

  Various techniques have been proposed for insolubilizing hazardous substances such as heavy metals contained in contaminated soil, etc., and suppressing or preventing these harmful substances from eluting from the soil. For example, it has been proposed that an insolubilizing agent containing magnesium oxide has excellent insolubilizing performance against elution of harmful substances (for example, Patent Document 1 and Patent Document 2). When an insolubilizing agent containing magnesium oxide is used, harmful substances such as heavy metals that could not be insolubilized by the cement solidification method can be insolubilized, and the pH of the insolubilized object to be treated becomes about 10, so that in particular, amphoteric metals such as lead It is known that the insolubilization performance is excellent (for example, Patent Document 3).

  The toxic substances mentioned here are toxic substances such as heavy metals such as lead, arsenic, hexavalent chromium, fluorine, mercury and cadmium, boron and selenium. In general, lead, mercury and cadmium in soil are often present in the form of a cation, and arsenic, hexavalent chromium, fluorine, boron and selenium are often present in the form of an anion.

In general, clay minerals in soil have a positive charge and a negative charge, but generally have a negative charge as a whole. This is because when the Si ⁴⁺ contained in the clay mineral is replaced with Al ³⁺ , the positive charge is insufficient, and the clay mineral as a whole is negatively charged. Even when the Al ³⁺ is replaced with Mg ²⁺ , the negative is similarly applied. This is because charging occurs. Moreover, negative charge may be generated due to dissociation of silanol on the surface of soil particles.

In the soil, humic substances are present in addition to clay minerals, and the constituents have a functional group such as -OH and -COOH, and dissociate hydrogen ions (H ⁺ ) depending on the pH and show negative charge. It is known.

  Cations are adsorbed and retained on clay minerals and humic substances in such negatively charged soil. Cationic species retained in the natural state are often alkali metals and alkaline earth metals such as sodium, potassium, magnesium and calcium, although they vary depending on the sedimentary environment and soil conditions. Here, cations such as calcium, magnesium, and potassium adsorbed on the negatively charged soil are easily exchanged with other cations. Cations (bases) that are adsorbed and retained by negatively charged soil and can be easily exchanged with other cations are collectively referred to as exchangeable bases or replaceable bases, and each cation is exchanged. Calcium, exchangeable magnesium, and exchangeable potassium.

Among harmful substances, heavy metals that exist in the form of cations, such as lead and cadmium, have the property of being easily adsorbed by the soil. For example, lead and cadmium are often present in soil after being adsorbed and retained as divalent cations such as Pb ²⁺ and Cd ²⁺ . On the other hand, arsenic, hexavalent chromium, fluorine, boron, and selenium are present in the soil in the form of anions and have the property of being difficult to be adsorbed by soil particles. For example, arsenic often exists as a trivalent or pentavalent anion such as AsO ₃ ³⁻ or AsO ₄ ³⁻ , and fluorine usually exists as a monovalent anion as F ⁻ .

In general, it is known that a magnesium oxide composition can be applied to suppression of elution from soil containing an anionic harmful substance such as arsenic and fluorine (for example, Patent Document 2 and Patent Document 4). However, even when arsenic and fluorine contaminated soils with different soil characteristics are insolubilized with magnesium oxide composition, the amount of arsenic and fluorine harmful substances eluted from untreated contaminated soil is similar. When the amount of the magnesium oxide composition added varies depending on the characteristics of the contaminated soil, that is, the insolubilizing effect varies depending on the characteristics of the contaminated soil, and in-situ insolubilization may be difficult to employ. In general, if the amount of magnesium oxide composition added exceeds 200 kg / m ³ , the material cost required for insolubilization increases, so it is difficult to obtain the advantage that the original insolubilization is low cost, and excavation removal Are often processed.

JP 2003-225640 A Japanese Patent Laid-Open No. 2004-298741 JP 2007-105549 A JP 2006-167524 A

  In view of the above problems, the present invention provides a method for treating arsenic-contaminated soil that can be treated at low cost by selecting an appropriate treatment method according to the characteristics of the contaminated soil.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that when a soil contaminated with harmful substances existing in the form of anions such as arsenic is insolubilized with a magnesium oxide composition, the insolubilizing effect is the soil. Acquired the knowledge that it depends on the characteristics (type of clay minerals, soil particle size composition, humus contained in the soil, etc.), and the insolubilization effect of arsenic by the magnesium oxide composition is interchangeable among soil characteristics. The present inventors have found that the amount of calcium is remarkably affected and thus completed the present invention.

  That is, when the present inventors insolubilize arsenic-contaminated soil, the elution of arsenic tends to be insolubilized below the soil elution amount standard in the contaminated soil where the exchangeable calcium amount per 100 g of dry soil is 110 mg or less. It has been found that the composition can be insolubilized without adding a large amount and the material cost can be reduced. On the other hand, when the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg, it becomes difficult to insolubilize below the soil elution amount standard, and the soil portion should be excavated and removed without adding the magnesium oxide composition. It has been found that the processing cost can be reduced.

That is, the present invention is as follows.
[1] A method suitable for treating arsenic-contaminated soil exceeding the soil elution standard, excavating and removing the contaminated soil portion where the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg, and per 100 g of dry soil The contaminated soil treatment method is characterized in that a contaminated soil portion having an exchangeable calcium content of 110 mg or less is insolubilized in situ using a magnesium oxide composition.
[2] The amount of arsenic elution in the contaminated soil portion to be insolubilized using the magnesium oxide composition is more than 1 time and not more than 10 times the soil elution amount standard, and the contaminated soil portion is treated with 30 to 250 kg / mg of magnesium oxide composition. The above-mentioned arsenic-contaminated soil treatment method, wherein m ³ is treated.
[3] The contaminated soil portion to be insolubilized using the magnesium oxide composition has an ignition loss of 9% by mass or less, an SiO ₂ content of 60 to 80% by mass, and an Al ₂ O ₃ content of 10 to 19% by mass. The arsenic contaminated soil treatment method.
[4] The contaminated soil portion to be insolubilized with the magnesium oxide composition contains 10-30% by mass of gravel, 10-80% by mass of sand, and 10-62% by mass of fine particles. Contaminated soil treatment method.
[5] The arsenic-contaminated soil treatment method described above, wherein the magnesium oxide composition has an MgO content of 75% by mass or more and a BET specific surface area of 10 to 50 m ² / g.

  According to the method for treating contaminated soil of the present invention, it is possible to significantly reduce the amount of arsenic eluted from contaminated soil that elutes arsenic exceeding the soil elution amount standard stipulated in the Soil Contamination Countermeasures Law at low cost. , Environmental load and economic load can be reduced.

It is a graph which shows the relationship between the amount of exchangeable calcium (Ca) of soil (mg / 100g of dry soil) and the reduction rate (%) of the arsenic elution amount of the soil to which the magnesium oxide composition is applied.

Hereinafter, embodiments of the present invention will be described in detail.
The present invention excavates and removes the contaminated soil portion where the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg, and uses the magnesium oxide composition for the contaminated soil portion where the exchangeable calcium amount per 100 g of dry soil is 110 mg or less. An arsenic-contaminated soil treatment method characterized by performing insolubilization treatment in situ. The present invention can also insolubilize anionic harmful substances such as fluorine, hexavalent chromium, boron, selenium, etc., but particularly shows an excellent effect on arsenic in anionic harmful substances. . Furthermore, the contaminated soil treatment method of the present invention can be applied to complex contaminated soil containing cations such as lead and cadmium in addition to anionic harmful substances such as arsenic.

  Magnesium oxide composition is added to and mixed with soil contaminated with anionic harmful substances such as arsenic, thereby adsorbing anionic harmful substances to magnesium oxide particles and hydrated magnesium hydroxide particles, Arsenic-contaminated soil can be effectively insolubilized by forming a hardly soluble salt (for example, magnesium arsenate) by reaction of dissolved magnesium ions with anionic harmful substances.

  On the other hand, as described above, soil particles are often negatively charged due to the type of clay minerals and functional groups of humic substances. For this reason, cations such as calcium and magnesium are electrically attracted to the soil particles, but the bonds are not strong and are easily replaced by other cations. In general, cation substitution is considered to be more easily adsorbed as a cation having a larger charge, and more likely to be adsorbed as a ionic radius is larger, but actually, it varies depending on soil environmental conditions.

  There are many details about the mechanism of insolubilization of arsenic-contaminated soil by magnesium oxide composition, but the effect of magnesium oxide composition on insolubilization by magnesium oxide composition varies depending on soil environmental conditions and soil characteristics. This is probably due to the reason. Magnesium ions supplied from the magnesium oxide composition are exchanged with calcium ions adsorbed and held on the soil particles, and chemically react with arsenate ions or arsenite ions from contaminated soil, thereby It is thought that elution is suppressed. Here, a part of the exchangeable calcium adsorbed and retained on the soil particles is desorbed under conditions where desorption occurs due to anions (for example, sulfate ions) present in the insolubilized soil or pH change. Magnesium ions supplied from the magnesium oxide composition are easily adsorbed on the exchangeable calcium portion, and the magnesium ions consumed for insolubilization of arsenic are insufficient due to chemical reaction with arsenate ions or arsenite ions derived from contaminated soil. Therefore, it is considered that the insolubilizing effect of the magnesium oxide composition is reduced.

  In a contaminated soil in which the amount of exchangeable calcium per 100 g of dry soil is a specific value, that is, the amount of exchangeable calcium exceeds 110 mg per 100 g of dry soil, a part of the exchangeable calcium in the soil particles is easily detached, It is considered that the effect of insolubilization of the magnesium oxide composition tends to be reduced because it is easily affected by the exchange adsorption. The influence of exchangeable calcium in contaminated soil is presumed to be more susceptible to divalent magnesium ions than monovalent sodium ions and potassium ions.

  In the present invention, a contaminated soil portion in which the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg is excavated and removed. Thus, the processing cost can be reduced by removing the contaminated soil portion in which the insolubilizing effect of the magnesium oxide composition is reduced without removing the excessively added magnesium oxide composition. The treatment method for excavated and contaminated contaminated soil is not particularly limited, but there are methods for removing contaminants by combining washing, classification, separation, etc. outside the site, heavy metals by injecting water and chemicals in situ, etc. And the like, and the like. After purification by washing treatment, it may be backfilled in uncontaminated soil or recycled as a clay-based raw material for cement. Furthermore, the soil portion where the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg is mixed with soil with a low exchangeable calcium amount on site, thereby reducing the exchangeable calcium amount as a whole to 110 mg or less, and insolubilization treatment. It is also possible to do.

  In the present invention, a contaminated soil portion having an exchangeable calcium amount of 110 mg or less per 100 g of dry soil is insolubilized in situ using a magnesium oxide composition. In the treatment method of the present invention, the contaminated soil portion to be insolubilized in situ can be adjusted to 110 mg or less by mixing soils having different exchangeable calcium amounts. For example, when a soil part A having an exchangeable calcium amount of 80 mg per 100 g of dry soil and a soil part B having an exchangeable calcium amount of 190 mg are mixed, the soil part A is 80% by mass and the soil part B is 20%. Using a magnesium oxide composition after adjusting the amount of exchangeable calcium to 110 mg or less by uniformly mixing mass% in advance with a self-propelled soil conditioner with excellent clay cutting and mixing performance, etc. It is also possible to insolubilize in situ.

  In the present invention, a contaminated soil portion having an exchangeable calcium amount of 110 mg or less can be treated by insolubilization. However, as a method of performing insolubilization in situ, an insolubilization backfilling measure is performed, and then the soil environmental conditions are changed. It is preferable to cover the part where the insolubilized backfilling measure is applied so as not to cover it or asphalt pavement. The insolubilized backfill measures are conducted after excavating contaminated soil with backhoe, etc., and measuring and separating the amount of exchangeable calcium for each soil, and the contaminated soil portion with an exchangeable calcium amount of 110 mg or less and the magnesium oxide composition. Mixing and stirring to insolubilize harmful substances and backfill the soil in place. As the curing period until the soil is insolubilized and backfilled, the soil portion that is insolubilized by mixing the contaminated soil portion with a exchangeable calcium content of 110 mg or less and the magnesium oxide composition is cured for 7 days or more after the insolubilization treatment. Preferably, it is more preferably cured for 14 days or more after the insolubilization treatment, and further preferably for 28 days or more after the insolubilization treatment.

  The magnesium oxide contained in the magnesium oxide composition used in the present invention preferably has a high hydration activity, and since hard-burned magnesium oxide has poor hydration activity, it may contain light-burned magnesium oxide. preferable. Magnesium oxide can be sufficiently used as long as it is a commercially available lightly burned magnesium oxide.

  As a starting material for magnesium oxide, it is preferable to use magnesium hydroxide that can be fired at a lower temperature than magnesium carbonate in order to obtain a composition having a large BET specific surface area. Generally, since the dehydration region temperature of magnesium hydroxide is relatively low at about 400 to 550 ° C., it is preferable to calcine at a higher temperature, and the firing temperature is preferably 550 to 700 ° C., more preferably 550 to 500 ° C. It is 650 degreeC, More preferably, it is 550-600 degreeC. The firing time is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, and further preferably 20 to 40 minutes.

  Magnesium hydroxide is naturally produced as bluestone, but most of it is seawater magnesia synthesized from seawater. Magnesium oxide can be obtained by adding lime milk to magnesium ions contained in sea water to precipitate magnesium hydroxide and calcining the precipitated magnesium hydroxide at a low temperature.

  The magnesium oxide composition used in the present invention preferably has a magnesium oxide (MgO) content of 75% by mass or more. The MgO content of the magnesium oxide composition is more preferably 80% by mass or more, still more preferably 85% by mass or more, and particularly preferably 87% by mass or more. When the MgO content in the magnesium oxide composition is 75% by mass or more, magnesium ions derived from MgO can chemically react with arsenate ions or arsenite ions derived from contaminated soil, thereby improving the insolubilizing effect of arsenic. .

The magnesium oxide composition used in the present invention preferably has a BET specific surface area of 10 to 50 m ² / g. As the BET specific surface area of the magnesium oxide composition increases, the amount added to the contaminated soil can be reduced, and the treatment cost can be further reduced. BET specific surface area of magnesium oxide composition used in the present invention is more preferably 20 to 50 m ² / g, more preferably from 30 to 50 m ² / g. When the BET specific surface area of the magnesium oxide composition is 20 to 50 m ² / g, the hydration activity of the magnesium oxide composition can exert an insolubilizing effect on arsenic contained in the soil, and the addition amount is reduced. Thus, the processing cost can be reduced. When the BET specific surface area of the magnesium oxide composition is less than 10 m ² / g, the hydration activity of the magnesium oxide composition becomes low, and the effect of insolubilization to arsenic may be insufficient, and the amount added is excessive. At the same time, the insolubilization rate may be slow, which is not preferable. When the BET specific surface area exceeds 50 m ² / g, the fluidity of the powder or slurry is lowered, and the workability of the insolubilizer may be deteriorated, which is not preferable.

  In the magnesium oxide composition used in the present invention, calcium carbonate, limestone powder, silica stone powder, blast furnace slag, steelmaking slag, calcium sulfide, silica, fly ash, as long as it does not impair the insolubilization performance of magnesium oxide inherent harmful substances Bentonite, vermiculite, hydrotalcite, hydrocalumite, hard calcined magnesia, dead calcined magnesia, magnesium hydroxide, magnesium carbonate, perlite, diatomaceous earth, zeolite, sepiolite, apatite, attapulgite, activated carbon, white carbon, alumina cement, It can be arbitrarily mixed with various additives such as chelate and iron powder.

  The treatment method of the present invention is preferably applied to arsenic-contaminated soil in which the arsenic elution amount is more than 1 time and not more than 10 times the soil elution amount standard. More preferably, it is preferably applied to contaminated soil having an arsenic elution amount of 6 times or less of the soil elution amount standard, and more preferably applied to a contaminated soil having an arsenic elution amount of 5 times or less of the soil elution amount standard. Among these, the treatment method of the present invention is suitable as a treatment method for naturally-derived arsenic-contaminated soil in which the amount of arsenic eluted exceeds the soil dissolution amount standard by a small amount. If it is applied to contaminated soil with an arsenic elution amount exceeding 10 times the soil elution amount standard, the amount of magnesium oxide composition required for in-situ insolubilization becomes excessive, which is economically undesirable. Here, the soil elution standard for arsenic is 0.01 mg / L or less (Soil Contamination Countermeasures Law Enforcement Regulations, Appendix 3). For the measurement of arsenic elution from contaminated soil, a test solution was prepared in accordance with the Environmental Agency Notification No. 46 (August 23, 1991). JIS K0102 “Factory drainage test” The amount of arsenic elution can be measured according to “Method”.

  The contaminated soil portion to be insolubilized with the magnesium oxide composition according to the present invention preferably has an ignition loss of 9% by mass or less, more preferably an ignition loss of 8.8% by mass or less, and further preferably an ignition loss of 8.7%. It is below mass%. If the loss on ignition of the contaminated soil is 9% by mass or less, there is little humic substance with negative charge of -OH or -COOH, which is adsorbed and retained by the cation in the soil, and the exchange is electrically attracted to the negative charge. The amount of soluble calcium is reduced and a sufficient insolubilizing effect is obtained.

The contaminated soil portion to be insolubilized using the magnesium oxide composition according to the present invention preferably has a SiO ₂ content of 60 to 80% by mass and an Al ₂ O ₃ content of 10 to 19% by mass, more preferably a SiO ₂ content. There 60 to 78 wt% and _Al 2 _{O 3} content is 12 to 19 wt%, more preferably SiO ₂ content is 60-76 wt% and _Al 2 _{O 3} content is 14 to 19 mass%. When the SiO ₂ content and the Al ₂ O ₃ content of the contaminated soil to be insolubilized are within the above ranges, the Si ⁴⁺ that generates a negative charge contained in the clay mineral in the contaminated soil is reduced in Al ³⁺ and negatively charged. The amount of exchangeable calcium that is electrically attracted decreases, and a sufficient insolubilizing effect is obtained.

  The contaminated soil portion to be insolubilized using the magnesium oxide composition according to the present invention includes gravel content of 10 to 30% by mass, sand content of 10 to 80% by mass, and fine particle content of 10 to 62% by mass. Is preferred. The contaminated soil portion preferably contains 12-25% by mass of gravel, 15-75% by mass of sand, and 15-62% by mass of fine particles, 14-20% by mass of gravel, and sand It is more preferable that the content contains 20 to 67% by mass and the fine particle content contains 18 to 62% by mass. When the gravel, sand and fine particles in the contaminated soil are within the above range, it indicates that there are relatively few negatively charged clay minerals in the contaminated soil, and exchangeability that is electrically attracted to the negative charge. The amount of calcium is reduced, and a sufficient insolubilizing effect is obtained.

The amount of magnesium oxide composition added to contaminated soil that elutes arsenic exceeding the soil elution standard is determined by the type of contaminated soil to be treated (the type of clay mineral that constitutes the soil, the size of the soil, the humus contained in the soil) Etc.) and the degree of contamination as described above, and is not particularly limited, but the magnesium oxide composition is added in an amount of ³⁰ to 250 kg / m ³ to the soil of the contaminated soil portion to be insolubilized. By doing so, a sufficient insolubilizing effect can be obtained. The addition amount of the contaminated soil portion to be insolubilized with the magnesium oxide composition is preferably 40 to 220 kg / m ³ , more preferably 50 to 200 kg / m ³ , and particularly preferably 70 to 200 kg / m ³ . If the added amount of the magnesium oxide composition is less than 30 kg / m ³ , the mixing of the insolubilizing agent and the soil may become insufficient, such being undesirable. On the other hand, if the added amount of the magnesium oxide composition exceeds 250 kg / m ³ , the processing cost becomes too high, which is not economically preferable. In addition, it is preferable to determine the addition amount of a magnesium oxide composition by the result of the previous indoor compounding test, and / or the result of the compounding test using an on-site mixer.

  The addition of the magnesium oxide composition to the contaminated soil portion can be applied in either a powder state or a slurry state. Mixing of magnesium oxide composition and contaminated soil is based on backhoes, backhoes with mixing buckets, stabilizers, self-propelled soil conditioners, stationary mixers, trencher type agitation mixers, deep mixing processors, power blenders, plant mixing, etc. A method using a commonly used mixer can be applied.

  EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

[Production of simulated contaminated soil]
In this experiment, five types of sample soils (A to E) having different particle size configurations, chemical compositions, cation exchange capacities, and the like were used. Each sample soil was heated at 40 ° C. to adjust the water content to about half of the natural water content. Next, an aqueous solution in which a predetermined amount of arsenic reagent is dissolved in water corresponding to the amount of evaporated water is prepared, added to the sample soil, and then kneaded at a low speed for 2.5 minutes with a soil mixer, and the soil adhering to the container or paddle is then mixed. Then, the mixture was further mixed for 2.5 minutes at low speed, and then cured for 7 days in a state sealed with a polyethylene bag to prepare a simulated contaminated arsenic soil. The arsenic simulated contaminated soil was prepared by adding a predetermined amount of disodium hydrogen arsenate heptahydrate (Na ₂ HAsO ₄ · 7H ₂ O, manufactured by Wako Pure Chemical Industries, Ltd.). A test solution was prepared in accordance with the Environmental Agency Notification No. 46 (August 23, 1991). The heavy metal concentration of the test solution was measured according to JIS K 0102 “Factory drainage test method”.

  The elution amount of the arsenic simulated contaminated soil was adjusted to about 5 to 6 times the soil elution amount standard (0.01 mg / L) described in Appendix 3 of the Enforcement Rules of the Soil Contamination Countermeasures Law.

[Soil property test]
(1) Measurement of particle size The gravel content, sand content and fine particle content of the sample soil were measured in accordance with JIS A 1204: 2009 “Soil particle size test method”.
(2) Measurement of water content The water content of the sample soil was measured in accordance with JIS A1204: 2009 "Soil water content test method".
(3) pH
The pH of the sample soil was measured according to JGS 0211-2009 “pH test method for soil suspension”.
(4) Chemical Composition The chemical composition of the sample soil was measured according to JIS R 5202: 2010 “Chemical Chemical Analysis Method”. Further, the loss on ignition of the sample soil was measured in accordance with JIS A1226: 2009 “Soil ignition loss test method”.
(5) Amount of exchangeable calcium The calcium concentration of the test solution prepared in accordance with the fertilizer analysis method 5.31.1 “cation exchange capacity” was measured by the atomic absorption method to obtain the amount of exchangeable calcium.
(6) Amount of exchangeable magnesium The magnesium concentration of the test solution prepared in accordance with the fertilizer analysis method 5.31.1 “cation exchange capacity” was measured by atomic absorption spectrometry to obtain the amount of exchangeable magnesium.
(7) Amount of exchangeable sodium The sodium concentration of the test solution prepared in accordance with the fertilizer analysis method 5.31.1 “cation exchange capacity” was measured by atomic absorption method to obtain the amount of exchangeable sodium.
(8) Amount of exchangeable potassium The potassium concentration of a test solution prepared in accordance with the fertilizer analysis method 5.31.1 “cation exchange capacity” was measured by an atomic absorption method to obtain the amount of exchangeable potassium.

[Insolubility test]
As described above, a magnesium oxide composition (BET specific surface area: 29 m ² / g) was added to the prepared arsenic-contaminated soil, kneaded at a low speed for 2.5 minutes with a soil mixer, and then adhered to the container or paddle. The soil was scraped off and kneaded at a low speed for 2.5 minutes. The amount of magnesium oxide composition added at this time was 75 to 200 kg / m ³ . The treated soil thus obtained was filled in a φ5 × 10 cm mold in three layers to prepare a cylindrical specimen, and hermetically sealed at 20 ° C. until the age of 7 days. A test solution was prepared in accordance with the Environmental Agency Notification No. 46 (August 23, 1991). The heavy metal concentration of the test solution was measured according to JIS K 0102 “Factory drainage test method”. In addition, since the lower limit of quantification of arsenic was 0.002 mg / L, when the measured value was less than 0.002 mg / L, it was expressed as 0.001 mg / L for convenience (Examples 1 to 12, Comparative Examples 1 to 8). ).

[Chemical composition of magnesium oxide]
Table 2 shows the chemical composition of the magnesium oxide composition used in the insolubilization test. The chemical composition of magnesium oxide was measured according to JIS M 8853: 1998 “Chemical analysis method of aluminosilicate raw material for ceramics”.
[BET specific surface area of magnesium oxide]
The BET specific surface area of the magnesium oxide composition used in the insolubilization test was measured by a constant volume gas adsorption method using a high-accuracy gas adsorption device (BELSORP-mini manufactured by Nippon Bell Co., Ltd.).

  As shown in Table 5, the amount of exchangeable calcium in the soil was larger in the order of sample soil A <sample soil B <sample soil C <sample soil D <sample soil E. It can be seen that the sample soil E having the largest amount of exchangeable calcium has a relatively high fine particle content of 69.8% by mass and a high water content ratio of 43%. This is presumed to be because it contains a relatively large amount of negatively charged clay minerals. In addition, the sample soil D having an exchangeable calcium amount of 125 mg / 100 g of dry soil and an exchangeable calcium amount of more than 110 mg per 100 g of the dry soil has a relatively high fine particle content of 62.1% by mass, and has a moisture content ratio. Is as high as 35%. On the contrary, it can be seen that the sample soil A having the smallest amount of exchangeable calcium has a relatively small fine particle content of 18.4% by mass and a low water content of 10%. This is presumed to be due to the relatively low content of clay minerals having a negative charge.

As shown in Table 6 and FIG. 1, when arsenic-contaminated soil exceeding the soil elution standard is insolubilized with a magnesium oxide composition, the exchangeable calcium amount is 10 mg, 30 mg, and 104 mg per 100 g of dry soil, and the exchangeable calcium amount. In the soil of 110 mg / 100 g or less of dry soil, the reduction rate of arsenic elution amount is as high as 80% or more in any case where the magnesium oxide composition addition amount is 75 to 200 kg / m ³ , and the arsenic elution amount after insolubilization treatment Was below the soil elution standard (Examples 1 to 12). On the other hand, in the soil where the exchangeable calcium amount is 125 mg and 140 mg per 100 g of dry soil, and the exchangeable calcium amount exceeds 110 mg / 100 g of dry soil, the reduction rate of arsenic elution is greatly reduced, and the magnesium oxide composition is reduced to 200 kg / m2. ^Even when increased to ³ , the arsenic elution amount after treatment could not satisfy the soil elution amount standard (Comparative Examples 1 to 8).

  According to the present invention, the amount of arsenic eluted from contaminated soil can be significantly reduced at low cost, and the environmental load and economic load can be reduced, which is industrially useful.

Claims (4)

The contaminated soil portion where the exchangeable calcium amount per 100 g of dry soil exceeds 110 mg is excavated and removed, the exchangeable calcium amount per 100 g of dry soil is 110 mg or less , the loss on ignition is 9 mass% or less, and the SiO ₂ content is 60 A method for treating arsenic-contaminated soil, comprising subjecting a contaminated soil portion having a content of ˜80 mass% and an Al ₂ O ₃ content of 10 to 19 mass% to insolubilization in situ using a magnesium oxide composition. The amount of arsenic elution in the contaminated soil portion to be insolubilized using the magnesium oxide composition is more than 1 time and less than 10 times the soil elution amount standard, and this contaminated soil portion is treated with magnesium oxide composition 30 to 250 kg / m ³ . The arsenic-contaminated soil treatment method according to claim 1, wherein treatment is performed. The contaminated soil portion to be insolubilized using the magnesium oxide composition contains 10 to 30% by mass of gravel, 10 to 80% by mass of sand, and 10 to 62% by mass of fine particles. Arsenic contaminated soil treatment method. The magnesium oxide composition has an MgO content of 75% by mass or more and a BET specific surface area of 10 to 50 m. ² The arsenic-contaminated soil treatment method according to any one of claims 1 to 3, which is / g.

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