JP2017012956A - Arsenic-containing soil cleaning method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an arsenic-containing soil cleaning method having the reduced waste amount to be generated.SOLUTION: Provided is an arsenic-containing soil cleaning method at least having the first to third steps in this order: the first step where arsenic-containing soil and a cationic cleaning agent selected from a chitosan series and a polyacrylic acid ester series are mixed and stirred in water; the second step where a solid phase and a liquid phase including arsenic are subjected to solid-liquid separation; and the third step where the arsenic in the liquid phase is treated.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for purifying arsenic-containing soil.

  Arsenic is widely present in rocks and soils in nature. When civil engineering work or shield excavation is carried out in a deposit area where arsenic is present, a large amount of soil and mud containing arsenic is generated. Since these are naturally-occurring pollutions, the soil content standard “150 mg or less of arsenic per kg of soil” in the Japanese soil pollution control law is satisfied, but “arsenic of 0. The soil elution amount standard of “being 01 mg or less” may be exceeded. For this reason, waste soil and mud containing arsenic are treated by an insolubilization method or a cleaning method.

  FIG. 2 shows a method for treating arsenic-containing soil by an insolubilization method. The insolubilization method is a method in which a flocculant is added to mud water containing arsenic, and arsenic is coagulated and precipitated together with suspended substances such as soil particles, followed by solid-liquid separation to recover arsenic in the solid phase. In the insolubilization method, since arsenic precipitates as a solid phase together with suspended solids, a large amount of soil containing arsenic is generated. And it is necessary to dispose of this soil appropriately at a contaminated soil treatment facility or a final disposal site.

FIG. 3 shows a method for treating arsenic-containing soil by a cleaning method. In the cleaning method, mud containing arsenic is washed with an acidic or alkaline cleaning agent, coarse particles are allowed to settle while arsenic is dissolved in the liquid, and then solid-liquid separation is performed. Arsenic is removed by adsorption. In Patent Document 1, soil contaminated with arsenic is washed with an acidic washing solution, arsenic is transferred from the soil to the washing solution, and the soil and the washing solution are separated into solid and liquid, and then an oxidant is added to the washing solution to remove arsenic. A method for precipitating and removing arsenic from the cleaning liquid by solid-liquid separation has been proposed. In Patent Document 2, arsenic in contaminated soil is alkali-extracted, then a polymer flocculant is added to agglomerate only soil particles having a particle size of 5 to 10 μm or more to form flocs. A method has been proposed in which purified soil is obtained by neutralization and stabilization treatment, and a concentrated contaminated soil containing arsenic is obtained by further coagulating and precipitating supernatant water containing fine particles of less than 5 to 10 μm and dissolved arsenic. ing.
In the cleaning method, a precipitate of coarse particles is recovered by solid-liquid separation, but since this precipitate does not contain arsenic, general disposal is possible. Arsenic-containing waste is generated when arsenic is removed from the liquid phase after solid-liquid separation, but there is an advantage that the amount of waste can be greatly reduced as compared with the insolubilization method.
However, since the cleaning method uses a highly acidic or strongly alkaline cleaning agent having a high cleaning effect, there is a high risk of chemicals to be handled, and the solid phase after solid-liquid separation must be neutralized after cleaning. The solid phase immediately after the solid-liquid separation does not contain free arsenic, but since it contains a strongly acidic or strongly alkaline washing solution, arsenic may be re-eluted before neutralization. In addition, since washing is performed with strong acidity or strong alkalinity, metal species other than arsenic and organic compounds also dissolve in the liquid phase. When arsenic is coagulated and precipitated from this liquid phase, other metal species and organic compounds are also precipitated, so that a large amount of precipitates with a low arsenic concentration is generated. When arsenic is removed from the liquid phase by adsorption, a large amount of adsorbent is required because other metals are also adsorbed. Furthermore, the liquid phase after solid-liquid separation is suspended in fine particles, and if passed through the adsorbent as it is, the adsorbent is clogged, so it is necessary to remove the fine particles in advance with a centrifuge or the like.

JP 2012-40544 A JP 2012-81386 A

  The present invention provides an arsenic-containing soil purification method that generates a small amount of waste.

1. An arsenic-containing soil purification method comprising at least the following first to third steps in this order.
The first step of mixing and stirring the arsenic-containing soil and the cationic purifier selected from chitosan or polyacrylate ester in water The second step of solid-liquid separating the solid phase and the liquid phase containing arsenic Third step of processing arsenic in the phase.
2. The first step is performed under conditions of pH 6.0 to 8.0. The arsenic-containing soil purification method according to claim 1.
3. In the first step, the concentration of the cationic purifier is in the range of 4 to 40 mg / L. Or 2. The arsenic-containing soil purification method according to claim 1.
4). The third step is a coprecipitation method. ~ 3. The arsenic-containing soil purification method according to any one of the above.
5. The third step is an adsorption method. ~ 3. The arsenic-containing soil purification method according to any one of the above.

  The arsenic-containing soil purification method of the present invention aggregates and precipitates suspended substances during soil washing, but arsenic does not aggregate and precipitate. When the solid phase and the liquid phase are subjected to solid-liquid separation, the arsenic remains in the liquid phase, and the solid phase does not contain arsenic. Therefore, the solid phase can be disposed of in general. If the soil is washed neutral, neutralization of the solid phase after solid-liquid separation is unnecessary. Since the solid phase after the separation is neutral, arsenic is not re-eluted from the solid phase. Further, the neutral liquid phase has a small amount of dissolution of metals and organic compounds other than arsenic. When the liquid phase after solid-liquid separation of the present invention is processed by the reprecipitation method, the resulting precipitate contains arsenic at a high concentration, and the content of other metals and organic compounds is low, so that the volume of waste can be greatly reduced. it can. In addition, the liquid phase containing arsenic does not contain fine particles and is transparent, and even if it passes through the adsorbent as it is, the adsorbent does not clog and can be easily removed without centrifugation. . Furthermore, since the amount of metal components other than arsenic is small, the amount of adsorbent can be reduced.

A method for treating arsenic-containing soil according to the present invention. A method for treating arsenic-containing soil by insolubilization. A method for treating soil containing arsenic by a cleaning method.

FIG. 1 shows a method for treating arsenic-containing soil according to the present invention.
The arsenic-containing soil purification treatment method of the present invention,
A first step of mixing and stirring the soil containing arsenic and a cationic purifying agent selected from chitosan-based or polyacrylic acid ester-based in water;
A second step of solid-liquid separation of the solid phase and the liquid phase containing arsenic,
A third step of treating arsenic in the liquid phase,
At least in this order.

  The present invention is characterized by using a cationic purification agent selected from chitosan-based or polyacrylic acid ester-based as a purification agent for soil containing arsenic. By using a specific cleaning agent, the suspended substance can be aggregated and precipitated while arsenic is dissolved in water. In addition, you may use together a chitosan type | system | group cleaning agent and a polyacrylic acid ester type | system | group cleaning agent.

The chitosan type | system | group purification agent used by this invention can be used without a restriction | limiting especially if it melt | dissolves in water and shows the coagulation ability with respect to a suspended substance. For example, chitosan having a degree of deacetylation of 40 to 90 mol% can be used. If the degree of deacetylation is less than 40 mol%, the solubility in water is poor, and those having a degree of deacetylation greater than 90 mol% are expensive because it takes time to produce from chitin. Since the main chain is cleaved, the weight average molecular weight is lowered.
The weight average molecular weight of chitosan is preferably 1,000 to 3,000,000, more preferably 3,000 to 2,000,000, and still more preferably 10,000 to 1,500,000. . The higher the weight average molecular weight, the greater the number of amino groups per molecule, so that the ability to agglomerate particles in the muddy water is excellent, and the suspended matter can be agglomerated and precipitated in a small amount. However, when the weight average molecular weight increases, it becomes difficult to dissolve in water and the handleability decreases.
Chitosan may be partially substituted with a polyoxyalkylene group, an acyl group, a sugar side chain, or the like, as long as it can be used as a cleaning agent of the present invention. By substituting with these, solubility in water can be improved. Examples of the polyoxyalkylene group include a polyethylene glycol group, and examples of the carboxyl group include an acetyl group and a propionyl group. Chitosan may be used alone or in combination of two or more different in molecular weight and degree of deacetylation.

The polyacrylic acid ester cleaning agent used in the present invention can be used without particular limitation as long as it dissolves in water and exhibits a coagulation ability with respect to a suspended substance. The polyacrylic acid ester cleaning agent can be produced by polymerizing a cationic monomer. At this time, other monomers may be copolymerized. Examples of the cationic monomer include dialkylaminoalkyl acrylates such as dimethylaminoethyl acrylate, diethylaminoethyl acrylate and diethylamino-2-hydroxypropyl acrylate, dialkylaminoalkyl acrylamide hydrochlorides such as dimethylaminopropyl acrylamide, and sulfates. Tertiary amine salts can be used. These cationic monomers can be used alone or in admixture of two or more.
The weight average molecular weight of the polyacrylic ester cleaner is preferably 1,000,000 to 10,000,000, more preferably 3,000,000 to 7,000,000, and 4,000,000 to 6,000,000 is more preferred. Moreover, it is preferable that a 0.25% solution viscosity is 100-4000 mPa * s (25 degreeC).

  A cationic cleaning agent selected from chitosan-based or polyacrylic ester-based has a cationic functional group such as an amino group, and the cationic functional group is ionized in water to be positively charged. Suspended substances dispersed in the muddy water are usually negatively charged and repelled, and do not aggregate due to the repulsion. When a positively charged flocculant is added to the muddy water, the negatively charged substance and the positively charged flocculant are neutralized to form a complex, and the repulsion is weakened to agglomerate. Precipitate. In the acidic solution, more cationic functional groups are ionized, so that the aggregation ability is increased. The smaller the pH, the more cationic functional groups that are positively charged, and the amount of purifier used can be reduced. However, if the pH is too low, the acidity is strong, the subsequent neutralization process is troublesome, and the danger increases. In addition, the amount of metal species other than arsenic and organic compounds dissolved in the liquid phase increases in the liquid phase. Therefore, it is preferable to use the cationic cleaning agent of the present invention under neutral conditions of pH 6.0 to 8.0. Further, the concentration of the cationic cleaning agent is not particularly limited as long as it is a concentration at which suspended particles are precipitated, but it is usually within a range of 4 to 40 mg / L.

Next, the arsenic-containing soil purification method of the present invention will be described in order.
"First step of mixing and stirring the soil containing arsenic and a cationic purifier selected from chitosan or polyacrylate ester" in water.
A soil containing arsenic and a cationic purifying agent selected from chitosan-based or polyacrylate-based are mixed and stirred in water. Examples of arsenic-containing soil include soil discharged during civil engineering work, dispersed soil in water, and muddy water generated during shield excavation work. The soil containing arsenic is mixed with a cationic purification agent in the state of soil, mud, mud and the like. The cationic cleaning agent is mixed with soil containing arsenic in a solid or aqueous solution. When mixed in a solid state, it takes time to dissolve in water, so mixing as an aqueous solution is preferable.
When the arsenic-containing soil and the cationic cleaning agent are mixed and stirred in water, the negatively charged suspended particles in the mud are neutralized by the positively charged cationic cleaning agent and do not repel and aggregate. Precipitates into a solid phase.
When the mixing and stirring of the arsenic-containing soil and the cationic purifier is performed under neutral conditions of pH 6.0 to 8.0, neutralization is not necessary. In addition, since metal species other than arsenic and organic compounds are difficult to dissolve under neutral conditions and do not shift to the liquid phase, the amount of waste containing arsenic generated after processing the liquid phase can be reduced. In the case of naturally-derived soil, the concentration of heavy metals other than arsenic, such as cadmium and lead, usually does not become a problem.

"Second step of solid-liquid separation of solid phase and liquid phase containing arsenic"
The solid phase precipitated in the first step and the liquid phase containing arsenic are subjected to solid-liquid separation. The solid-liquid separation method is not particularly limited, and a sedimentation separator, a belt press, a screw press, a pressure dehydrator, a centrifugal dehydrator, or the like can be used. When the first step is performed under neutral conditions of pH 6.0 to 8.0, acid resistance is not required for the apparatus used for solid-liquid separation, so that an inexpensive apparatus can be used. Since the solid phase after solid-liquid separation is purified soil that does not contain arsenic, general treatment is possible as treated soil below the soil content standard and soil elution standard in the Soil Contamination Countermeasures Law. is there. Furthermore, the neutralization process is unnecessary when the pH of the solid phase is 6.0 to 8.0.

"Third process for treating arsenic in liquid phase"
Arsenic in the liquid phase can be treated by coprecipitation or adsorption.
In the coprecipitation method, arsenic is removed from the liquid phase by adsorbing arsenic to the precipitate and coprecipitating the metal in the flocculant as a metal oxide. The flocculant can be used without particular limitation as long as it contains a metal species that adsorbs and precipitates arsenic, and includes iron, manganese, aluminum, calcium, magnesium, silica, etc., specifically Examples thereof include polyferric sulfate, polyferric chloride, ferric sulfate, ferric chloride, polyaluminum chloride, polyaluminum sulfate, alum, and polysilica iron. These flocculants are also used as flocculants in conventional insolubilization methods.
In the present invention, when the first step is carried out under neutral conditions of pH 6.0 to 8.0, metal species and organic compounds other than arsenic are hardly dissolved in the liquid phase. The concentration of compounds other than arsenic contained in the precipitated solid phase is low, and the volume of waste that requires proper treatment as an arsenic-containing material can be greatly reduced.

In the adsorption method, a liquid phase containing arsenic is passed through an adsorbent, and the adsorbent is adsorbed and removed. The adsorbent is not particularly limited as long as it can adsorb arsenic, and iron hydroxide, activated alumina, cerium hydroxide and the like can be used.
In the present invention, the particles in the mud are coagulated and precipitated in the second step, and the liquid phase is transparent and does not contain fine particles. Therefore, the adsorbent is not clogged even if it passes through the adsorbent as it is without centrifuging the solid-liquid separated liquid phase. Further, when the first step is performed under neutral conditions of pH 6.0 to 8.0, the amount of metal species other than arsenic contained in the liquid phase is small, and the adsorbent mainly adsorbs arsenic. The amount of adsorbent necessary for arsenic adsorption and the replacement frequency can be reduced, and the amount of adsorbent discarded can be reduced.
Next, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

After adding 1.85 kg of water to 5 kg of soil containing arsenic and mixing, the mixture was passed through a sieve having an opening of 75 μm, and further diluted 5 times to obtain simulated mud water in which the soil containing arsenic was dispersed.
"Example 1"
While stirring 500 ml of this simulated mud at 130 rpm, 2% aqueous solution of chitosan-based purifying agent (manufactured by Fuji Engineering Co., Ltd., trade name: Fujiclean) is adjusted to 2 mg / L, 10 mg / L, and 20 mg / L, respectively. In addition, the mixture was further stirred at 130 rpm for 1 minute and then allowed to stand for 5 minutes.
The supernatant after standing was filtered with a 0.45 μm filter, and the turbidity was measured using a turbidimeter (manufactured by Hanna Instruments Japan, apparatus name: HI 98703). Moreover, the arsenic density | concentration in the supernatant liquid after filtration was measured with the inductively coupled plasma mass spectrometer (ICP-MS).

"Example 2"
The same procedure as in Example 1 was performed except that a polyacrylic acid ester-based cleaner (manufactured by MT Aquapolymer Co., Ltd., trade name: Aron Flock C-508UL) was used instead of the chitosan-based cleaner.
“Comparative Example 1”
It was carried out similarly to the said Example 1 except having used the polymethacrylic-ester type | system | group cleaning agent (The product made from MT Aqua Polymer Co., Ltd., brand name: Aron Flock C-300) instead of the chitosan-type cleaning agent.
“Comparative Example 2”
It was carried out similarly to the said Example 1 except having used the polyethyleneimine type | system | group cleaning agent (The Hayashi Chemical Co., Ltd. make, brand name: EB-a) instead of the chitosan type | system | group cleaning agent.
Table 1 shows the turbidity and arsenic concentration of Examples 1 and 2 and Comparative Examples 1 and 2. Table 1 shows the turbidity and arsenic concentration of the supernatant of the simulated mud water as controls.

None of the cationic cleaners used in Examples 1 and 2 and Comparative Examples 1 and 2 precipitated arsenic in the simulated mud water, and the arsenic concentration in the liquid phase was almost the same as the control. Examples 1 and 2 using the cationic purification agent of the present invention are excellent in the ability to aggregate and precipitate suspended substances in simulated mud water, and the turbidity of the supernatant liquid decreases as the concentration of the purification agent increases. did. On the other hand, Comparative Examples 1 and 2 were inferior in the ability to aggregate and precipitate suspended substances, and the turbidity of the supernatant liquid was high and cloudy. From this result, the concentration of the purifying agent of the present invention is preferably 4 to 40 mg / L.
When the cationic cleaning agent of the present invention is used, only suspended substances can be precipitated without precipitating arsenic in the liquid phase, the solid phase after solid-liquid separation does not contain arsenic, and the general treatment of the solid phase Is confirmed to be possible.

The arsenic concentration of the simulated muddy water used above is lower than the arsenic concentration in the contaminated water generated in the actual field. The following experiment was conducted assuming contaminated water generated at the actual site.
After adding 3.7 kg of water to 10.0 kg of soil containing arsenic, the mixture was passed through a sieve having an opening of 75 μm, and further diluted 5 times to obtain simulated contaminated water.
"Example 3"
While stirring 500 ml of this simulated contaminated water at 130 rpm, a 2% aqueous solution of a chitosan-based purifying agent (manufactured by Fuji Engineering Co., Ltd., trade name: Fujiclean) is added to 10 mg / L, and further stirred at 130 rpm for 3 minutes. And left for 5 minutes.
In the same manner as in Example 1, turbidity and arsenic concentration were measured.
“Comparative Example 3”
Instead of the chitosan-based cleaner, poly ferric sulfate (manufactured by Nankai Chemical Co., Ltd.) was added in the same manner as in Example 3 except that it was added to 200 mg / L and 400 mg / L, respectively.
“Comparative Example 4”
It carried out similarly to the said Example 3 except having added liquid polyaluminum chloride (the Daimei Chemical Co., Ltd. make, brand name: Thai pack) so that it might become 200 mg / L instead of a chitosan type | system | group purifier.
The turbidity and arsenic concentration of Example 3 and Comparative Examples 3 and 4 are shown in Table 2. Table 2 shows the turbidity and arsenic concentration of the supernatant of the simulated contaminated water as controls.

  In Example 3 using the cationic cleaning agent of the present invention, it was confirmed that the turbidity was lowered and the arsenic concentration was maintained. The ferric sulfate used in Comparative Example 3 has a high ability to adsorb arsenic, and a decrease in the arsenic concentration was confirmed prior to the decrease in turbidity. Arsenic could not be detected from the supernatant after the suspended material aggregated and precipitated, and all the arsenic was transferred to the solid phase. In Comparative Example 4 using polyaluminum chloride, the coagulation ability was inferior to that of Example 3, and the turbidity was less likely to decrease. In addition, the arsenic concentration was lower than in Example 3, suggesting that arsenic tends to migrate into the solid phase.

Claims (5)

An arsenic-containing soil purification method comprising at least the following first to third steps in this order.
The first step of mixing and stirring the arsenic-containing soil and the cationic purifier selected from chitosan or polyacrylate ester in water The second step of solid-liquid separating the solid phase and the liquid phase containing arsenic Third step of processing arsenic in the phase.   The arsenic-containing soil purification method according to claim 1, wherein the first step is performed under conditions of pH 6.0 to 8.0.   The arsenic-containing soil purification method according to claim 1 or 2, wherein in the first step, the concentration of the cationic purification agent is in the range of 4 to 40 mg / L.   The arsenic-containing soil purification method according to any one of claims 1 to 3, wherein the third step is a coprecipitation method. The arsenic-containing soil purification method according to any one of claims 1 to 3, wherein the third step is an adsorption method.