JP4433812B2 - Soil purification method and soil purification agent - Google Patents

Description

  The present invention relates to a soil purification method for purifying soil or the like made of viscous soil contaminated with heavy metals, and a soil purification agent for use in such a soil purification method.

In recent years, there has been a rapid increase in cases where a large amount of contaminated soil is carried to final disposal sites. Since it is expensive to carry out contaminated soil to the disposal site and the remaining capacity of the disposal site is rapidly decreasing, a method for cleaning and refilling the contaminated soil has been proposed.
In particular, for soil contaminated with heavy metals, classification cleaning treatment is performed by dividing the excavated soil into a coarse particle having a particle size of 75 μm or more and a fine particle having a particle size of 75 μm or less by wet classification with water. Has been applied. Here, the fine particles are carried out to the disposal site, but the coarse particles are backfilled as healthy soil (see, for example, Patent Document 1).
JP 2002-254063 A

  However, the classification cleaning is difficult to apply to soil containing 20% or more of fine particles, such as viscous soil. In the case of cohesive soil, the washing efficiency is not sufficient with water alone, and the cost for disposal is high, so there is a possibility that even if the coarse particles are backfilled, it is not profitable.

  On the other hand, a method of washing contaminated soil composed of coarse particles to which heavy metals are attached with a strongly acidic or strongly basic solution has been studied. If this method is used to wash viscous soil, there is a risk that even heavy metals that cannot be eluted when soil coexists with water. The action of a strong acid or strong base on the soil may be stronger than the action of treating the collected soil according to the measurement method defined by the Minister of the Environment based on, for example, the Ordinance for Enforcement of the Soil Contamination Countermeasures Law (Ministry of Environment No. 29 of 2002). Therefore, viscous soil that has been subjected to strong acid or strong base treatment may not satisfy environmental standards based on, for example, the Soil Contamination Countermeasures Law (Act No. 53 of 2002) and cannot be backfilled. In addition, the viscous soil that has been subjected to such treatment is not limited to environmental standards, and may not satisfy standards specialized for soil use and the like.

  The present invention has been made in view of the above problems, and the object of the present invention is to provide a soil made of viscous soil contaminated with heavy metals at high efficiency and at low cost, for example, an environment based on the Soil Contamination Countermeasures Law. An object of the present invention is to provide a method of purifying so as to satisfy a predetermined standard such as a standard.

In order to solve the above problems, the present invention provides a soil purification method for purifying soil by washing soil made of viscous soil contaminated with hexavalent chromium with an aqueous solution, wherein the aqueous solution contains sodium sulfate. It is either a dissolved aqueous solution or a weakly basic aqueous solution in which sodium carbonate is dissolved and the hydrogen ion exponent increased by hydrolysis of the sodium carbonate is reduced with dilute sulfuric acid. .

Furthermore, the soil purification agent of the present invention is used when purifying soil consisting of viscous soil contaminated with hexavalent chromium with an aqueous solution, and an aqueous solution in which sodium sulfate is dissolved or sodium carbonate is dissolved in the sodium carbonate. It is any one of the aqueous solutions which show weak basicity which reduced the hydrogen ion index | exponent which became high by hydrolysis of (3) with the dilute sulfuric acid .

According to the soil purification method and the soil purification agent of the present invention, a soil composed of viscous soil contaminated with hexavalent chromium is highly efficient and low-cost, for example, a predetermined standard such as an environmental standard based on the Soil Contamination Countermeasures Law. Can be purified to meet. Moreover, both the purification soil and the soil purification agent can be reused.

  Hereinafter, an embodiment of a soil purification method according to the present invention will be described with reference to FIG.

  In this embodiment, a soil purification method for excavating soil composed of viscous soil contaminated with heavy metals and cleaning the excavated soil, wherein the aqueous solution for cleaning the soil is an inorganic compound. An aqueous solution in which a salt is dissolved is used.

FIG. 1 is a flowchart showing the steps of a soil purification method according to an embodiment of the present invention. First, approximately 20 m ^{3 of} hexavalent chromium-contaminated soil is excavated, and an initial elution amount of hexavalent chromium to water described later is shown. Is measured, a part of the excavated soil is collected (S100).

  Next, a cleaning agent is added to the excavated soil and stirred with, for example, a mixer or the like to obtain muddy water. Agitation by a mixer or the like is performed in a batch system, for example, and muddy water is stored in, for example, an under tank. In this embodiment, this cleaning agent corresponds to an aqueous solution for cleaning the soil excavated from the soil.

  The present invention is not limited to this embodiment, and depending on the nature and amount of inorganic compound salts, contaminated soil, heavy metals, etc., by adding acid or alkali separately, The hydrogen ion index (pH) may be adjusted, and the hydrogen ion index (pH) of mud water after adding the cleaning agent may be adjusted. Here, the hydrogen ion index (pH) is approximately 3.0 to 11.0, and preferably approximately 4.0 to 7.0 or approximately 7.0 to 10.0.

  Thus, by washing the contaminated soil by adjusting the hydrogen ion index (pH) to be neutral, weakly acidic, or weakly basic, for example, when the soil coexists with water It is possible to suppress elution of heavy metal ions that cannot be performed. Moreover, the soil washed with such a neutral, weakly acidic, or weakly basic aqueous solution can be backfilled in the soil without any treatment such as neutralization.

Thereafter, the cleaning agent is added to the muddy water stored in the under tank so as to have a predetermined liquid-solid ratio, and is stirred, for example, with an underwater mixer (S102). A suitable flocculant made of silica fume or a polymer substance is added to the agitated mud water to cause it to agglomerate to obtain a sedimented and concentrated mud water (sludge) (S104). This improves the muddy water dewatering rate. At this time, heavy metal containing hexavalent chromium is removed from the clean water by an appropriate heavy metal removing device, and the clean water is recovered as a cleaning agent in step S102.
Further, rinsing water is added to the sludge and stirred with an underwater mixer (S106). An appropriate flocculant made of silica fume or a polymer substance is added to the liquid which has been agitated with rinse water and becomes muddy water again, and agglomerated to form sludge again (S108). At this time, heavy metal containing hexavalent chromium is removed from the clean water by an appropriate heavy metal removing device, and the clean water is recovered as rinse water in step S106.

  Next, the sludge is dehydrated and separated into solid and liquid by, for example, filter pressing. A part of the solid-liquid separated water is collected for measuring the concentration of hexavalent chromium described later (S110). The heavy metal containing hexavalent chromium is removed from the solid-liquid separated water at this time by an appropriate heavy metal removing device, and the solid-liquid separated water is recovered as rinse water in step S106.

  The dehydrated cake generated by the above dehydration is hermetically stored in, for example, a pre-comp pack. A part of the dehydrated cake is collected for measuring the amount of hexavalent chromium to be dissolved in water, which will be described later. If, for example, the measurement result of the elution amount of the hexavalent chromium is lower than an environmental reference value (predetermined value) described later, this dehydrated cake is effectively used as a backfill material as purified soil (S112). If the elution amount of hexavalent chromium from the soil collected in step S112 to the water is higher than the environmental reference value, the process returns to step S106 to re-rinse the soil.

  In the present embodiment, the metal element that contaminates the soil is hexavalent chromium, but the present invention is not limited to this. For example, soil contaminated with metal elements such as cadmium (Cd), lead (Pb), mercury (Hg), arsenic (As), and selenium (Se) may be washed with a cleaning agent.

  Examples of the present invention will be described below.

In this example, the soil of Kanto Loam containing 100 to 200 mg / kg of hexavalent chromium (Cr ⁶⁺ ), the particle size distribution in the soil is about 15.2% with a particle size of 75 μm to 2 mm (sand), about The soil with 51.4% particle size of 5 μm to 75 μm (silt) and approximately 33.4% particle size of 5 μm or less (clay) was targeted for purification.

In addition, an aqueous solution in which sodium sulfate (Na ₂ SO ₄ ), which is a salt of an inorganic compound, is dissolved, or sodium carbonate (Na ₂ CO ₃ ), which is a salt of an inorganic compound, is dissolved and becomes higher by hydrolysis of the sodium carbonate. An aqueous solution in which the hydrogen ion index (pH) was lowered with dilute sulfuric acid (H ₂ SO ₄ ) was used as a cleaning agent.

  Here, the concentration of sodium sulfate is approximately 0.05 mol / L, and the pH of the cleaning agent is approximately 6.0 to 7.0 (weakly acidic). The concentration of sodium sulfate is preferably about 0.01 to 0.1 mol / L, more preferably about 0.05 to 0.1 mol / L. Further, the concentration of sodium carbonate is approximately 0.01 mol / L, but the pH of the cleaning agent is approximately 8.0 to 9.0 (weakly basic) by the addition of sulfuric acid.

  Further, using the soil collected in steps S100 and S112, the amount of hexavalent chromium eluted when the hexavalent chromium in the soil is eluted in water is measured, and the hexavalent chromium in the solid-liquid separated water collected in step S110 is measured. The concentration, that is, the hexavalent chromium removing power of the cleaning agent in this example was measured.

  Hexavalent chromium elution amount refers to the preparation and analysis methods (diphenylcarbamate) specified by the environmental standards for soil contamination based on the regulations of the Ordinance for Enforcement of the Soil Contamination Countermeasures Law (Ministry of the Environment Ordinance No. 29). It is the concentration of hexavalent chromium in the test solution measured by the zido absorptiometry).

Table 1 shows the hexavalent chromium removing power of the cleaning agent in this example, and the hexavalent chromium elution amount obtained by the analysis using the above-described sample preparation method and analysis method.

  Here, the concentration of hexavalent chromium in the “solid-liquid separated water” in Table 1 corresponds to the hexavalent chromium removing power of the cleaning agent, and the concentration of hexavalent chromium in the “eluent of purified soil” in Table 1 is Corresponds to the elution amount of hexavalent chromium from soil to water. Moreover, in Table 1, the data of hexavalent chromium when the soil in a present Example was wash | cleaned with water are also given for the comparison.

As shown in Table 1, the concentrations of hexavalent chromium in the detergent in which two salts (Na ₂ SO ₄ and Na ₂ CO ₃ + H ₂ SO ₄ ) were dissolved were 0.20 mg / L and 0.35 mg, respectively. / L is higher than the concentration of hexavalent chromium in the water by washing with water of 0.14 mg / L.
Therefore, it can be said that the hexavalent chromium removing power of each of the two salt cleaning agents in this example is larger than the hexavalent chromium removing power of water.

In addition, as shown in Table 1, by the above-described sample preparation method, two salts (Na ₂ SO ₄ and Na ₂ CO ₃ + H ₂ SO ₄ ) were each eluted from the soil washed with the detergent dissolved therein. The concentration of hexavalent chromium is 0.03 mg / L and less than 0.01 mg / L.

  Here, since the environmental standard value concerning the hexavalent chromium contamination of soil based on the provisions of the Soil Contamination Countermeasures Law (Act No. 53 of 2002) is 0.05 mg / L, the cleaning agent in this example was used. It can be said that the cleaned soil satisfies the environmental standards.

  Furthermore, according to Table 1, it can be said that only water washing of the soil is insufficient to satisfy such environmental standards (the amount of hexavalent chromium eluted by water washing is 0.07 mg / L).

By the way, compared with the case where the detergent in which the two salts (Na ₂ SO ₄ and Na ₂ CO ₃ + H ₂ SO ₄ ) in this example are dissolved is water alone, the hexavalent chromium removing power and the six The reason why it is excellent in both the amount of valent chromium elution is that the exchange action between SO ₄ ²⁻ ions and anion CrO ₄ ²⁻ as a hexavalent chromium contaminant is likely to occur. It can also be considered that such exchange action is induced by the similarity in atomic structure between the exchanged anions (SO ₄ ²⁻ and CrO ₄ ²⁻ ).

  If the soil is purified using the cleaning agent, hexavalent chromium can be removed with high efficiency, and the elution amount of hexavalent chromium from the purified soil into water can be easily made lower than the environmental standard value. Therefore, low-cost and low-load type soil remediation is possible even for viscous soil such as Kanto Loam.

Further, in this example, since no strong acid or strong basic detergent is used, the soil after purification can be reused without particularly performing neutralization treatment or the like. Furthermore, since salts (Na ₂ SO ₄ and Na ₂ CO ₃ + H ₂ SO ₄ ) dissolved in the cleaning agent are not active against, for example, reducing iron powder, hexavalent chromium is reduced by the iron powder. Can be processed and reused cleaning agents.

Next, in this example, Kanto Loam soil (elution amount 0.2 mg / L, content 100 mg / kg) made of viscous soil contaminated with hexavalent chromium (Cr ⁶⁺ ) was targeted for purification.

  The contaminated soil was washed with each cleaning agent for 7 minutes, then agglomerated with a polymer flocculant, further solid-liquid separated by pressure dehydration, and then purified as a solid. The amount of elution when hexavalent chromium in the soil was eluted in water was measured. Further, the concentration of hexavalent chromium in the solid-liquid separated water, that is, the hexavalent chromium removing power of the cleaning agent in the present embodiment was measured (Ministry of the Environment Notification No. 13).

Table 2 shows the analysis based on the hexavalent chromium removing power of the cleaning agent in this example (the concentration of hexavalent chromium in “solid-liquid separation water”) and the above-described sample preparation method and analysis method (diphenylcarbazide absorptiometry). The elution amount of hexavalent chromium obtained in this manner (the concentration of hexavalent chromium in the “cleaned soil eluate”). Data for washing with water and alkali (NaOH) are also shown for comparison.

  As shown in Table 2, when water is used as a cleaning agent, the hexavalent chromium concentration in the solid-liquid separated water is as low as 0.10 mg / L, and the removal amount of hexavalent chromium is small. Moreover, even if compared with the hexavalent chromium elution amount (0.2 mg / L) before washing, since a high value was not shown, a sufficient washing effect was not observed.

  On the other hand, when alkali was used as the cleaning agent, the hexavalent chromium concentration in the solid-liquid separation water was as high as 0.20 mg / L, and the removal of hexavalent chromium was possible. Since the elution amount of hexavalent chromium in the eluate of soil was as high as 0.18 mg / L and exceeded the environmental standard value (0.05 mg / L), it was unsuitable as a cleaning agent.

In contrast, when sulfate (Na ₂ SO ₄ ) and carbonate (Na ₂ CO ₃ ) were used as cleaning agents, the hexavalent chromium concentration in the solid-liquid separated water was 0.23 mg / L. A high value was exhibited, and hexavalent chromium could be removed. Moreover, the hexavalent chromium elution amount of the purified soil eluate (treated soil) both showed a low value of 0.04 mg / L, and none of them exceeded the environmental standard value (0.05 mg / L).

  From this, when the salt of an inorganic compound (inorganic salt) is used as a cleaning agent, hexavalent chromium contained in the soil is selectively removed mainly in a state where it is easily eluted. However, at this time, even hexavalent chromium which is in a state of being difficult to elute is dared to be easily eluted, and it is understood that it is not removed by elution.

Therefore, it can be said that sulfate (Na ₂ SO ₄ ) and carbonate (Na ₂ CO ₃ ) are effective as cleaning agents.

It is a flowchart which shows the process of the soil purification method in one Embodiment of this invention.

Claims (2)

A soil purification method for purifying the soil by washing the soil made of viscous soil contaminated with hexavalent chromium with an aqueous solution,
The aqueous solution is either an aqueous solution in which sodium sulfate is dissolved or an aqueous solution exhibiting weak basicity in which sodium carbonate is dissolved and the hydrogen ion index increased by hydrolysis of the sodium carbonate is reduced with dilute sulfuric acid. soil purification method, characterized in that it. Used to purify soil made of viscous soil contaminated with hexavalent chromium with an aqueous solution, an aqueous solution in which sodium sulfate is dissolved, or a hydrogen ion index that is increased by hydrolysis of sodium carbonate by dissolving sodium carbonate A soil purification agent characterized by being any one of weakly basic aqueous solutions obtained by lowering the amount of sulfuric acid with dilute sulfuric acid .

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