JP6906330B2 - How to purify contaminated groundwater - Google Patents

Description

The present invention relates to a method for purifying contaminated groundwater, which purifies contaminated groundwater or prevents recontamination of groundwater.

Groundwater pollution by chemical substances may be regulated by environmental standards, etc., and many measures are being considered and implemented. Among such measures, a method of purifying the in-situ where groundwater pollution is occurring without significantly changing the current situation is particularly useful.

One of the effective methods for purifying groundwater pollution is a method of decomposing harmful substances using microorganisms (bioremediation). Various attempts have already been made to purify harmful chemical substances in groundwater by effectively utilizing the microorganisms existing in the in-situ soil. Among hazardous chemical substances, volatile organic chlorine compounds such as tetrachlorethylene and trichlorethylene (hereinafter sometimes referred to as "VOCs") have set groundwater environmental standards, and the technology for purifying them is highly required. It is a thing. In addition, nitrate nitrogen and nitrite nitrogen (hereinafter, may be referred to as "nitrate / nitrite nitrogen") have also set groundwater environmental standards, and techniques for purifying them are required. ..

As a technique for purifying groundwater pollution by VOCs and nitrate / nitrite nitrogen, a technique for purifying groundwater by anaerobic microorganisms is known. It purifies VOCs and nitrate / nitrite nitrogen by anaerobicizing the groundwater environment and providing hydrogen donors that provide energy sources for anaerobic microorganisms that decompose VOCs and nitrate / nitrite nitrogen. It is a technology.

For example, when a water-soluble organic compound is injected into the ground as an energy source for anaerobic microorganisms, oxygen is consumed by the aerobic microorganisms to form an anaerobic environment. As a result, anaerobic microorganisms are activated, and purification of VOCs and nitrate / nitrite nitrogen proceeds. Here, the water-soluble organic compound is a substance that serves as a nutrient source or a hydrogen donor for activating aerobic microorganisms and anaerobic microorganisms.

Since the water-soluble organic compound dissolves in water and easily flows out in the ground where the groundwater flow velocity is high, additional injection of the water-soluble organic compound is required until the purification is completed, which requires time, labor and cost. there were. Furthermore, even if the concentration of VOCs decreases, the VOCs contained in the surrounding soil may elute and cause contamination again. At this time, if the water-soluble organic compounds have disappeared, the sufficient anaerobic environment necessary for purification cannot be maintained, and it becomes difficult for anaerobic microorganisms to purify VOCs, so that groundwater pollution recurs ( Will rebound). Therefore, there has been a demand for a means for preventing recontamination of groundwater in a place where pollution may occur or where purification has been completed.

Various methods have been disclosed as techniques for purifying VOCs and nitrate / nitrite nitrogen using anaerobic microorganisms. Patent Document 1 is characterized in that a purifying agent is melted by heating, injected under pressure into contaminated soil or groundwater, diffused, solidified, and then decomposed by microorganisms grown under anaerobic conditions. In-situ purification methods for contaminated soil and / or groundwater pollution are disclosed.

Japanese Patent No. 3649294

In the in-situ purification method described in Patent Document 1, since the purifying agent is solidified in the soil, it is possible to maintain an anaerobic environment and prevent the recurrence of groundwater pollution. However, when the temperature of the applied environment is low, the temperature of the purifying agent drops rapidly and solidifies, which narrows the reach of the purifying agent from the injection well and reduces the area to be purified in the soil. It will be limited.

In order to maintain a long-term anaerobic environment in order to cope with highly permeable ground and to suppress rebound after purification, organic compounds that serve as nutrient sources for anaerobic microorganisms are injected into the ground in the form of solid powder. The method is valid. In particular, high molecular weight resins are suitable for the purpose of maintaining an anaerobic environment for a long period of time because their morphology does not easily collapse even in soil and they gradually decompose over time.

The solid powder organic compound to be injected is preferable because the smaller the particle shape, the wider the ground can be injected. On the other hand, since the surface area of the organic compound processed into a fine powder increases, it tends to aggregate and it becomes difficult to uniformly disperse it in water as a medium. In particular, the solid powder resin generally has a problem that it does not disperse in water when it is simply added to water, it aggregates and becomes non-uniform, and it floats on the water surface to significantly reduce handling.

The present invention has been made in view of the above circumstances. That is, the subject of the present invention is to provide a method for purifying contaminated groundwater, which can maintain a state of low pollution for a long period of time in a wide area in the soil, purify the contaminated groundwater, and further prevent recontamination of the groundwater. It is to be.

The present inventors follow the procedure of adding a water-soluble organic compound to a particulate hydrolyzable resin, stirring the mixture, then adding water to prepare a mixed solution, and then injecting the mixed solution into the soil. It has been found that this makes it possible to suppress the aggregation of the particulate hydrolyzable resin and provide the hydrogen donor to a wide area in the soil. It was also found that by using a water-soluble organic compound and a hydrolyzable resin in combination as a hydrogen donor, it is possible to maintain an anaerobic environment in the soil and suppress recontamination. ..

The present invention is based on such findings. That is, the present invention is a method for purifying contaminated groundwater by utilizing anaerobic microorganisms to purify contaminated groundwater or prevent recontamination of groundwater, and as a hydrogen donor, a particulate hydrolyzable resin. And a water-soluble organic compound are used in combination, the hydrolyzable resin is added to the water-soluble organic compound and stirred, then water is added to prepare a mixed solution, and then the mixed solution is injected into the soil. It is a method for purifying contaminated groundwater.

The pollutant in the contaminated groundwater may be at least one of a volatile organochlorine compound, nitrate nitrogen and nitrite nitrogen.
The water-soluble organic compound preferably contains at least one of an alcohol, an alcohol derivative, a carboxylic acid and a carboxylic acid derivative.
The hydrolyzable resin is preferably an aliphatic polyester.
The hydrolyzable resin preferably contains at least one of polylactic acid and polyoxalate.
The average particle size of the hydrolyzable resin is preferably 1 to 100 μm.
Further, a packer can be installed above the well for injecting the mixed solution into the soil, and the mixed solution can be injected into the soil using a pump.

The method for purifying contaminated groundwater of the present invention can purify contaminated groundwater or prevent recontamination of groundwater by maintaining a state of low pollution for a long period of time in a wide area in the soil.

It is a figure which showed the particle size distribution by the dynamic light scattering method of the particle of a hydrolyzable resin. It is a figure which shows the particle size distribution of a hydrolyzable resin in a mixed solution. It is a figure which showed the peak distribution of the primary particle and the secondary particle of a hydrolyzable resin in a mixed solution for each kind of a water-soluble organic compound to be added. It is a figure which showed the average particle diameter of the primary particle of a hydrolyzable resin in a mixed solution for each kind of a water-soluble organic compound to be added. It is a figure which showed the polydispersity index of the primary particle of a hydrolyzable resin in a mixed solution for each kind of a water-soluble organic compound to be added. It is a figure which showed the reach distance of the particulate hydrolyzable resin in the soil. It is a figure which showed an example of the specific method at the time of injecting a mixed solution into soil.

Embodiments of the present invention will be described in detail below. However, the embodiments described below are examples of embodiments of the present invention, and the present invention is not limited to these contents, and can be variously modified and implemented within the scope of the gist thereof.

The present embodiment is a method for purifying contaminated groundwater by utilizing anaerobic microorganisms to purify contaminated groundwater or prevent recontamination of groundwater. The pollutants in contaminated groundwater are not particularly limited, but among the pollutants, VOCs and nitrate / nitrite nitrogen have groundwater environmental standards. The purification method of this embodiment is suitable for contaminated groundwater containing VOCs or nitrate / nitrite nitrogen. Further, the purification method of the present embodiment is effective not only for purification of contaminated groundwater but also for purification of soil recontaminated with VOCs.

(Pollutants)
VOCs have traditionally been widely used industrially as solvents and cleaning agents. Specific examples of VOCs include tetrachlorethylene (PCE), trichlorethylene (TCE), cis-1,2-dichloroethylene (cis-1,2-DCE), 1,1-dichloroethylene (1,1-DCE), vinyl chloride ( VC or VCM), 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,2-dichloroethane, ethanetetrachloride, carbon tetrachloride, chloroform, dichloromethane and the like.

In the purification of contaminated groundwater containing VOCs, VOCs are dechlorinated using anaerobic microorganisms in an anaerobic environment to change them into chlorine-free compounds. Anaerobic microorganisms that degrade VOCs are activated by the presence of hydrogen donors in an anaerobic environment.

The main sources of nitrate and nitrite nitrogen are nitrogen fertilizer, livestock manure, and domestic wastewater. Some of these substances are converted to nitrate nitrogen through the action of microorganisms in the soil via ammoniacal nitrogen and then nitrite nitrogen. Nitrate and nitrite nitrogen are easily dissolved in water and are not easily retained in soil, so they have the property of easily dissolving in groundwater.

Purification of VOCs and nitrate / nitrite nitrogen by anaerobic microorganisms is carried out by the following steps.
(I) Even in the soil where anaerobic microorganisms are present, aerobic microorganisms are actively active in an aerobic environment. In order to activate anaerobic microorganisms, it is necessary to remove oxygen in the environment to create an anaerobic environment. To form an anaerobic environment, nutrient sources and hydrogen donors are injected into the soil to activate aerobic microorganisms and consume oxygen in a limited area of the soil.

(Ii) In an oxygen-free anaerobic environment, the presence of hydrogen donors activates the activity of anaerobic microorganisms. Nitrate and nitrite nitrogen are finally converted to harmless nitrogen by anaerobic denitrifying bacteria. In addition, VOCs are replaced with hydrogen by anaerobic dechlorinating bacteria and finally converted into harmless substances such as ethylene and ethane that do not contain chlorine.

(Water-soluble organic compound)
Water-soluble organic compounds are hydrogen-containing organic compounds that are metabolized by microorganisms to serve as nutrient sources and also function as hydrogen donors when substituting hydrogen atoms for chlorine atoms in VOCs. Examples of the water-soluble organic compound include alcohols, carboxylic acids, amines, esters and derivatives thereof. Further, the water-soluble organic compound preferably contains at least one of an alcohol, an alcohol derivative, a carboxylic acid and a carboxylic acid derivative. In particular, the water-soluble organic compound is preferably either a carboxylic acid or a carboxylic acid derivative, and the carboxylic acid derivative is more preferably a carboxylic acid salt. As the water-soluble organic compound, only a specific type may be used, or a plurality of types may be mixed and used. Further, the water-soluble organic compound is preferably an organic compound consisting of only a carbon atom and a hydrogen atom, or an organic compound composed of a carbon atom, a hydrogen atom and an oxygen atom. Further, the water-soluble organic compound is preferably an organic compound having a molecular weight low enough to be biodegraded so as not to remain in the soil.

Specific examples of the water-soluble organic compound are as follows. Examples of the alcohol include ethylene glycol, polyethylene glycol, glycerin and the like. Examples of the carboxylic acid include glycolic acid, lactic acid, malic acid, succinic acid, ascorbic acid, acetic acid and the like. Examples of the carboxylate include sodium lactate, sodium ascorbate, sodium succinate and the like. In addition, biodegradable nutrient sources such as yeast extract, soymilk, corn milk, peptone, beef tallow, coconut oil, and soybean oil may be added to the water-soluble organic compound.

(Hydrolytic resin)
The hydrolyzable resin is a resin that is hydrolyzed in the presence of water to generate a low molecular weight monomer. The generated monomer is an organic compound containing a carbon atom and a hydrogen atom like a water-soluble organic compound, and is metabolized by a microorganism to become a nutrient source and replaces a chlorine atom of VOCs with a hydrogen atom. It also functions as a hydrogen donor.

Since the hydrolyzable resin is a high-molecular-weight resin that is insoluble in water, it is not easily decomposed by chemical decomposition or microorganisms, remains in the contaminated ground for a long period of time, and continuously releases a hydrogen donor. It is possible. In addition, since it does not decompose rapidly, it does not increase the total organic carbon concentration (TOC) and has a small impact on the environment.

When a water-soluble organic compound and a hydrolyzable resin are used in combination, the water-soluble organic compound functions as a nutrient source or a hydrogen donor that acts on microorganisms in the short term, and the hydrolyzable resin acts as a nutrient source that acts on microorganisms in the long term. Or it functions as a hydrogen donor. When a water-soluble organic compound and a hydrolyzable resin are used in combination, an anaerobic environment can be maintained sustainably. Further, by changing the ratio of the water-soluble organic compound and the hydrolyzable resin, the balance of short-term and long-term activation of anaerobic microorganisms can be controlled. Further, since the hydrogen donor is also supplied from the hydrolyzable resin, the consumption of the water-soluble organic compound can be suppressed, and the injection amount of the water-soluble organic compound used in the entire purification operation can be reduced.

Furthermore, once the target area has been purified, after a lapse of time, VOCs and nitrate / nitrite nitrogen contained in the surrounding soil may elute, or new VOCs and nitrate / nitrate from the outside. Nitrite nitrogen may flow in or groundwater level may fluctuate, causing pollution again. Even in such a case, since the activity of anaerobic microorganisms is maintained in an anaerobic environment, it is possible to suppress the occurrence of recontamination.

By changing the weight average molecular weight (Mw) of the hydrolyzable resin, the period during which the hydrolyzable resin remains in the contaminated ground and releases the hydrogen donor can be adjusted. If the weight average molecular weight of the hydrolyzable resin is too small, it will be hydrolyzed in a short period of time, and it will be difficult to sustain it as a hydrogen donor for a long period of time. Therefore, the weight average molecular weight of the hydrolyzable resin is preferably 12,000 or more. Further, when the weight average molecular weight of the hydrolyzable resin is 12,000 or more, the hydrolyzable resin can be formed into a pellet shape or a powder shape to maintain the shape. In addition, since the hydrolysis rate differs depending on the type and pH of the hydrolyzable resin, even if the type and pH of the hydrolyzable resin are changed, the hydrolysis rate remains in the contaminated ground to release the hydrogen donor. Can be adjusted.

Specific examples of the hydrolyzable resin are polyesters, polyamides, polysaccharides, proteins and the like in which monomers are polycondensed by ester bonds, amide bonds, glycosidic bonds, peptide bonds and the like. Specific examples of the monomers constituting the polyester include oxalic acid, malonic acid, oxalic acid, glutaric acid, adipic acid, pimelli acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, and terephthalic acid. Dicarboxylic acids such as decandicarboxylic acid and cyclohexanedicarboxylic acid; polyhydric alcohols such as ethylene glycol, propylene glycol, butanediol, octanediol, dodecanediol, neopentyl glycol, glycerin, pentaerythritol, sorbitan, bisphenol A, polyethylene glycol; Hydroxycarboxylic acids such as glycolic acid, lactic acid, malic acid, hydroxypropionic acid, hydroxybutyric acid, hydroxyvaleric acid, hydroxycaproic acid, hydroxybenzoic acid; glycolide, caprolactone, butyrolactone, valerolactone, propiolactone, undecalactone, etc. Examples include lactones.

Specific examples of the monomers constituting the polyamide include dicarboxylic acids such as lactoic acid, adipic acid, sebacic acid, glutaric acid, decandicarboxylic acid, cyclohexhexanedicarboxylic acid, terephthalic acid, isophthalic acid, and anthracendicarboxylic acid; Diamines such as methylenediamine, nonanediamine, methylpentadiamine and phenylenediamine; lactams such as caprolactam, undecanelactam and lauryllactam can be mentioned. Specific examples of the monomers constituting the polysaccharide and protein include monosaccharides such as glucose, fructose and galactose; α-amino acids such as glutamic acid, aspartic acid, glycine and lysine.

Examples of polysaccharides and proteins that are polycondensed by glycosidic bonds and peptide bonds include starch, modified starch, cellulose, chitin, chitosan, gluten, gelatin, soybean protein, collagen, and keratin.

Among the hydrolyzable resins, an aliphatic polyester is preferable from the viewpoint of easy decomposition. Among the aliphatic polyesters, poly (α-hydroxy acid), poly (β-hydroxy alkanoate), poly (ω-hydroxy alkanoate), polyalkylene dicarboxylate and the like are preferable from the viewpoint of biodegradability. Specific examples of preferable hydrolyzable resins include polyglucoric acid, polylactic acid, poly (β-hydroxybutyrate), poly (β-hydroxyvaleric acid), poly-β-propiolactone, poly-ε-caprolactone, and polyethylene. Examples thereof include succinate, polybutylene succinate, polyethylene oxalate, and polybutylene oxalate. As these hydrolyzable resins, only a specific type may be used, or a plurality of types may be mixed and used. As the type of hydrolyzable resin, an appropriate type of hydrolyzable resin can be selected and used according to the soil quality of the ground to be purified.

The hydrolyzable resin preferably contains at least one of polylactic acid and polyoxalate because it has an appropriate hydrolysis rate and is excellent in handleability. Both polylactic acid and polyoxalate are biodegradable, consisting only of carbon and hydrogen atoms. Here, the polyoxalate means a copolymer such as polyethylene oxalate or polybutylene oxalate in which oxalic acid and a diol such as ethylene glycol or butylene glycol are copolymerized.

When polylactic acid and polyoxalate are used in combination, new effects can be exhibited as described below.
Polylactic acid produces lactic acid when hydrolyzed. Lactic acid is particularly useful as a nutrient source and hydrogen donor for microorganisms. However, since the rate of hydrolysis of polylactic acid in an anaerobic environment is not high, the amount of lactic acid generated may be insufficient when it is desired to activate the activity of anaerobic microorganisms.
On the other hand, polyoxalate has a higher hydrolysis rate than polylactic acid, and oxalic acid is generated by hydrolysis. This oxalic acid catalyzes the hydrolysis of polylactic acid.

When polylactic acid and polyoxalate are used in combination, oxalic acid is first generated by hydrolysis of polyoxalate, and when the concentration of oxalic acid increases, hydrolysis of polylactic acid is promoted to generate lactic acid, which causes microorganisms to grow. It acts effectively on activation. By changing the amount ratio of polylactic acid and polyoxalate, it is possible to control the elution rate and persistence of decomposition products relatively freely. In the method of using polylactic acid and polyoxalate in combination, the resin may be melt-mixed and used as a composite resin, or the particles of the respective resins may be mixed and used.

In the ground where the flow velocity of groundwater is high, even if the water-soluble organic compound is injected into the soil, the water-soluble organic compound dissolves in the groundwater and easily flows out together with the groundwater. That was difficult.
On the other hand, the hydrolyzable resin of the present embodiment is in the form of particles. When the hydrolyzable resin is in the form of particles, the hydrolyzable resin can be retained in the gaps between the soil particles. Further, when the hydrolyzable resin stays in the gaps between the soil particles, the gaps between the soil particles are closed, so that the flow velocity of the groundwater can be reduced. In other words, when a particulate hydrolyzable resin is used, the flow velocity of groundwater in the purification target area is reduced, and the hydrolyzable resin stays in the soil even in the ground where the flow velocity of groundwater is high, and the original hydrogen donor Can exert its function as. Further, when the hydrolyzable resin is in the form of particles, even if oxygen, nitrate ion, sulfate ion, etc. flow in as components of the inflowing groundwater, they are not subjected to an oxidizing action or the like to the inside of the particles. Not easily affected by.

The particle size of the hydrolyzable resin can be set to an appropriate particle size according to the soil quality of the ground to be purified. However, when a purifying agent containing a hydrolyzable resin is injected from an injection well in the soil, if the particle size of the hydrolyzable resin is small, the distance that the hydrolyzable resin reaches around the injection well due to the injection work will be long. It becomes longer and can purify a large area in the soil. Therefore, the average particle size of the hydrolyzable resin is preferably 1 to 100 μm, more preferably 1 to 10 μm. The particle size of the hydrolyzable resin can be measured by a dynamic light scattering method or the like. Further, the particles of the hydrolyzable resin can be formed into an appropriate shape according to the soil quality of the ground to be purified. The average particle size of the hydrolyzable resin is the average particle size of the primary particles of the hydrolyzable resin, and is usually measured as a 90% diameter (d90). The 90% diameter is the particle size at the point where the cumulative curve is 90% when the cumulative curve is obtained with the total volume of the particle population as 100%.

As described above, in the present embodiment, by using a water-soluble organic compound and a particulate hydrolyzable resin in combination as a hydrogen donor, it is possible to stably maintain an anaerobic environment for a long period of time. Is. As a result, even if VOCs and nitrate / nitrite nitrogen contained in the surrounding soil elute and cause recontamination, recontamination can be suppressed. Furthermore, even when purifying highly permeable ground, the particulate hydrolyzable resin functions as a material for closing the gaps between soil particles, suppressing the permeability of groundwater and consuming hydrogen donors. It can be suppressed and the sustainability of purification of groundwater contaminated by anaerobic microorganisms can be enhanced.

The composition ratio of the particulate hydrolyzable resin and the water-soluble organic compound is not particularly limited, and should be appropriately changed according to the characteristics of each component, the type of the target pollutant, the duration of the anaerobic environment, and the like. Can be done. As a typical composition, for example, the mixing ratio of the hydrolyzable resin and the water-soluble organic compound is 1: 1 to 3 by mass ratio.

Next, the procedure of the contaminated groundwater purification method of the present embodiment will be described.
The method for purifying contaminated groundwater of the present embodiment is carried out by injecting a purifying agent containing a water-soluble organic compound and a hydrolyzable resin into the soil of the contaminated ground.

Here, when the surface of the particulate hydrolyzable resin is hydrophobic, it floats on the surface of the water when mixed with water, or floats in the water as a non-uniform mass, and is uniform. It is difficult to make a good solution. Further, if the particle size of the hydrolyzable resin is small, the surface area is increased, so that it is easy to aggregate, and it becomes difficult to form secondary particles to form lumps and uniformly disperse them in water.

As a method of uniformly dispersing the particulate hydrolyzable resin in water, there is a method of adding a dispersant such as a surfactant. However, there are many types of dispersants that cannot be applied from the viewpoint of environmental protection. In addition, environmentally safe and highly biodegradable dispersants are expensive.

The present inventors have examined methods for solving the above problems, and have found that the following methods are excellent methods. First, as a first step, a step of adding a water-soluble organic compound to the particulate hydrolyzable resin and stirring the mixture is performed, and the water-soluble organic compound is blended on the surface of the particles of the hydrolyzable resin. This makes the surface of the hydrolyzable resin particles more compatible with water. Next, as a second step, when a step of adding water is performed, the particulate hydrolyzable resin is relatively easily dispersed in water. As a result, an aqueous solution in which the particles of the hydrolyzable resin are dispersed relatively uniformly is formed.

The stirring device used in the first step is not particularly limited, and a known mixer, blender or the like can be used. The conditions for stirring (stirring speed, stirring time, etc.) can also be set as appropriate.

By performing the above two steps in the production of the purifying agent, the aggregation of the particulate hydrolyzable resin is suppressed, the formation of secondary particles is suppressed, and the particulate hydrolyzable resin is uniformly contained in water. It became possible to disperse. The resulting mixed solution is then injected into the soil as a purifying agent. Since the particulate hydrolyzable resin is uniformly dispersed in the purifying agent, it is possible to supply the purifying agent having a uniform component to a wide area in the soil.

Further, in the above procedure, since it is not necessary to add a dispersant or the like as the third component, only the essential component is sufficient, which is simple and the manufacturing cost can be reduced.

To inject the purifying agent into the soil, a well with a general screen (opening) can be used as a well for soil injection. A water-soluble organic compound is mixed with a hydrolyzable resin and stirred, and the obtained mixture is added to a storage tank containing water and stirred to obtain a mixed solution. After that, an injection pump is provided between the pipe connecting the storage tank and the injection well, and a backflow prevention packer is installed at one upper part of the injection well. By pumping the mixed solution in the reservoir, the purifier is injected into the soil through the screen of the well. In this method, since the purifying agent is preferentially injected into a highly permeable place around the injection well, it is possible to efficiently reduce the flow velocity of the groundwater. The method of injecting the purifying agent into the contaminated ground is not particularly limited, and a known method such as a natural injection method from an existing injection well can be appropriately applied.
When purifying contaminated groundwater, water quality monitoring for quantifying the oxygen concentration, pollutant concentration, and residual amount of purifying agent in the groundwater is appropriately performed as necessary.

Hereinafter, the present invention will be specifically described with reference to test examples.
As the particulate hydrolyzable resin, a polylactic acid resin (weight average molecular weight 15,000) blended with 10% polyoxalate was used. FIG. 1 is a diagram showing a particle size distribution of particles of this hydrolyzable resin by a dynamic light scattering method. The particle size distribution was measured using a laser micron sizer LMS-2000e manufactured by Seishin Enterprise Co., Ltd.

When the cumulative curve is obtained with the total volume of the particle population as 100%, the particle size at the point where the cumulative curve is 90% is defined as 90% diameter (μm). The particles of this hydrolyzable resin had a 90% diameter of 23 μm. Therefore, the average particle size of the primary particles of this hydrolyzable resin was set to 23 μm, and the following studies were carried out.

The following were used as the water-soluble organic compounds.
Lactate: Kanto Chemical Co., Ltd., Reagent Sodium Lactate: Wako Pure Chemical Industries, Ltd., Reagent Sodium Ascorbate: Kanto Chemical Co., Ltd., Reagent Sodium Succinate: Kanto Chemical Co., Ltd., Reagent

2.0 g of the above water-soluble organic compound was added to 1.0 g of the particulate hydrolyzable resin, and the mixture was stirred using a mortar.

Next, 10 ml of water was added and further stirred, and the hydrolyzable resin and the water-soluble organic compound were dispersed in water while stirring with a magnetic stirrer to prepare a mixed solution.

The particle size distribution of the particles dispersed in the obtained mixed solution was measured using the following apparatus.
Measuring device: ELSZ-1000 manufactured by Otsuka Electronics Co., Ltd.

FIG. 2 is a diagram showing the particle size distribution of the hydrolyzable resin in the mixed solution. FIG. 2A shows, for comparison, a mixed solution when the particulate hydrolyzable resin is dispersed in water without adding a water-soluble organic compound (hereinafter, “the hydrolyzable resin is dispersed only in water”. The particle size distribution in the "mixed solution") is shown. FIG. 2B shows a mixed solution obtained by adding sodium lactate to a particulate hydrolyzable resin, stirring the mixture, and then adding water to disperse the mixture (hereinafter, “hydrolyzable resin containing sodium lactate”). The particle size distribution in the "mixed solution in which the above is dispersed" is shown.

As shown in FIG. 2A, in the case of a mixed solution in which the hydrolyzable resin is dispersed only in water, the ratio of secondary particles having a particle size larger than 23 μm (23,000 nm) is large, whereas the ratio is large. As shown in FIG. 2B, it can be seen that in the case of the mixed solution in which the hydrolyzable resin to which sodium lactate is added is dispersed, the ratio of the primary particles having a particle size smaller than 23 μm is large. That is, it can be seen that the aggregation of the primary particles is suppressed. Not all particles having a particle size smaller than 23 μm are primary particles, and statistically, some secondary particles are also mixed.

FIG. 3 is a diagram showing the peak distribution (relative ratio) of the primary particles and the secondary particles of the hydrolyzable resin in the mixed solution for each type of the water-soluble organic compound to be added. For comparison, the leftmost side of the figure shows the case of a mixed solution in which the hydrolyzable resin is dispersed only in water. It was found that the addition of the water-soluble organic compound decreased the ratio of the secondary particles and increased the ratio of the primary particles (first peak). That is, it was found that the aggregation of the primary particles was suppressed by preliminarily blending the hydrolyzable resin with the water-soluble organic compound.

FIG. 4 is a diagram showing the average particle size of the primary particles of the hydrolyzable resin in the mixed solution for each type of water-soluble organic compound to be added. For comparison, the leftmost side of the figure shows the case of a mixed solution in which the hydrolyzable resin is dispersed only in water. It was found that the addition of the water-soluble organic compound suppressed the aggregation of the primary particles and reduced the average particle size of the primary particles in the region of 23 μm or less.

FIG. 5 is a diagram showing the polydispersity index of the primary particles of the hydrolyzable resin in the mixed solution for each type of water-soluble organic compound to be added. For comparison, the leftmost side of the figure shows the case of a mixed solution in which the hydrolyzable resin is dispersed only in water. It was found that the addition of the water-soluble organic compound suppressed the aggregation of the primary particles and reduced the polydispersity index of the primary particles in the region of 23 μm or less. Here, the polydispersity index is a measure of the polydispersity of the particle size distribution, that is, the spread of the particle size distribution.

FIG. 6 is a diagram showing the reach of the particulate hydrolyzable resin in the soil. This figure shows the results of a model experiment on the permeability of particulate hydrolyzable resin in soil. A mixed solution in which the hydrolyzable resin was dispersed only in water and a mixed solution in which the hydrolyzable resin to which sodium lactate was added were dispersed, which were used in the above experiment, were prepared. A certain amount of each mixed solution was naturally permeated from the top of the glass column filled with silica sand. The distribution of the reach distance (cm) from the top of the hydrolyzable resin moved in the glass column and the amount of the moved resin (mg / g) is shown. FIG. 6A shows a mixed solution in which a hydrolyzable resin is dispersed only in water, and FIG. 6B shows a mixed solution in which a hydrolyzable resin containing sodium lactate is dispersed. be. It was found that the reach of the particulate hydrolyzable resin was extended by adding the water-soluble organic compound.

FIG. 7 is a diagram showing an example of a specific method for injecting the mixed solution into the soil. The underground soil composition consists of a pavement 20 on the surface, a buried soil layer 21 with a thickness of about 2.5 m below it, an aquifer 22 with a thickness of about 6.5 m below it, and a silt layer 23 below it. It has become. The aquifer 22 was mainly composed of sand, contained groundwater, and had a hydraulic conductivity k of ^10-6 to ^10-5 m / s. The silt layer 23 was a layer made of fine-grained soil mainly composed of cohesive soil, and had a hydraulic conductivity k of ^10-9 to ^10-8 m / s. The injection well 10 is installed to a depth reaching the silt layer 23, and the screen (opening) 11 is provided over the thickness of the aquifer 22 so that the purifying agent can be injected into the aquifer 22. Have.

As an injection device for the purifying agent into the soil, an in-vehicle injection device 1 mounted on a truck bed was used. In the in-vehicle injection device 1, a water-soluble organic compound is mixed with a hydrolyzable resin and stirred, and the obtained mixture is added to storage tanks 2A and 2B storing water and stirred, and a mixed solution for injection is added. Was prepared. The composition of the mixed solution was about 1 kg: about 2 kg: 1 kl of hydrolyzable resin: water-soluble organic compound: water. An injection pump 3, a flow meter 4, and a pressure gauge 5 are installed in the pipe 6 connecting the storage tanks 2A and 2B and the injection well 10. Further, a packer 12 for preventing backflow is installed at one upper part of the injection well 10.

Using the injection pump 3, 9 kl of the mixed solution was injected into the soil at about 40 l / min, and the injection pressure at the head of the pipe during that period was measured. As a result, the particulate hydrolyzable resin was supplied into the soil from the injection well 10 without remaining in the storage tanks 2A and 2B in the form of lumps. Throughout the injection period, the injection pressure was 0.0 MPa, no increase in the injection pressure was observed, and it was confirmed that the purifying agent could be easily injected around the injection well 10.

Claims (7)

A method of purifying contaminated groundwater that uses anaerobic microorganisms to purify contaminated groundwater or prevent recontamination of groundwater.
As a hydrogen donor, a particulate hydrolyzable resin and a water-soluble organic compound are used in combination.
The hydrolyzable resin contains at least one of polylactic acid and polyoxalate.
The average particle size of the hydrolyzable resin is 1 to 100 μm.
A method for purifying contaminated groundwater, which comprises adding the hydrolyzable resin to the water-soluble organic compound, stirring the mixture, then adding water to prepare a mixed solution, and then injecting the mixed solution into soil. The method for purifying contaminated groundwater according to claim 1, wherein the pollutant of the contaminated groundwater is at least one of a volatile organic chlorine compound, nitrate nitrogen and nitrite nitrogen. The method for purifying contaminated groundwater according to claim 1 or 2, wherein the water-soluble organic compound contains at least one of an alcohol, an alcohol derivative, a carboxylic acid and a carboxylic acid derivative. The method for purifying contaminated groundwater according to any one of claims 1 to 3, wherein the hydrolyzable resin is an aliphatic polyester. The method for purifying contaminated groundwater according to any one of claims 1 to 4, wherein the water-soluble organic compound is either a carboxylic acid or a carboxylic acid derivative. The method for purifying contaminated groundwater according to any one of claims 1 to 5, wherein the mixing ratio of the hydrolyzable resin and the water-soluble organic compound is 1: 1 to 3 in terms of mass ratio. The invention according to any one of claims 1 to 6, wherein a packer is installed above the well for injecting the mixed solution into the soil, and the mixed solution is injected into the soil using a pump. How to purify contaminated groundwater.

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