JP4218564B2 - Method for purification of contaminated aquifer using purification agent for water containing pollutant - Google Patents

Description

The present invention relates to a purification treatment method contaminated aquifer using a purification reaction agent water containing pollutants, particularly using purification reaction agent and the cleaning reagent water containing pollutants heavy metal contamination The present invention relates to a method for purifying an aquifer.

  Pollutants such as arsenic, cadmium, cyanide, lead, hexavalent chromium, mercury, selenium and other heavy metals adversely affect living organisms, especially in animals such as drinking water directly or contaminated grains, livestock, etc. Ingestion can cause neuropathy. Therefore, these heavy metals are also considered as heavy metals in the national guidelines for investigation and countermeasures related to soil and groundwater contamination (1999 Environmental Agency Guidelines).

  These heavy metals are widely used as manufacturing raw materials in the plating painting industry, the semiconductor manufacturing industry, the chemical industry, the battery manufacturing industry, etc. It may be washed away from a management warehouse and contaminate the soil as well as the surrounding groundwater. In addition, not only in the vicinity of these factories, but also in the old mine area, in recent years the final disposal site for waste and illegal dumping areas, etc., these heavy metals are likely to be washed away from places where the concentration of heavy metals tends to be high. It is coming.

  The mechanisms of groundwater contamination by these heavy metals that have a profound effect on the surrounding environment can be broadly classified as follows. Normally, hexavalent chromium, arsenic, cyanide, and selenium that are present in anions are relatively difficult to be absorbed by the soil, but they are often mixed with rainwater to reach the aquifer and often contaminate groundwater. . Lead, cadmium, mercury, and the like may also contaminate the aquifer when the aquifer groundwater surface around the contaminated soil is high or due to leakage due to damage to underground piping in factories and the like.

  For this reason, when groundwater is contaminated by these heavy metals or other contaminants, or there is a risk of contamination, a method that can quickly purify the contaminants by some means is required, and a method using a contaminant removal treatment facility is required. It is common.

  Methods for using the pollutant removal treatment facility include digging a pumping well, pumping up groundwater, removing the pollutant at the pollutant removal treatment facility, and discharging it to a river, or excavating contaminated soil. Then, there is a method of burying it in the original excavation site or sealingly burying it in the final disposal site even if it is transported to the pollutant removal treatment facility and subjected to purification treatment that removes the pollutant by heating and incineration or microorganisms. The latter method can intensively purify, but requires excavation of all contaminated soil, transportation of excavated soil and embedding after treatment.

  In addition, as a method that does not use the pollutant removal treatment facility, for example, metallic iron is present in the groundwater in the aquifer and the organic halogen compound is dehalogenated under a low oxygen concentration to purify the contaminated groundwater. Method, for example, as shown in Patent Document 1, the channel of the aquifer where the contaminated groundwater flows is excavated into a channel that reaches the impermeable layer below the aquifer to bury metal iron and contaminate it. Examples include a method of forming a permeation reaction wall for removing substances and purifying the groundwater when it passes through the permeation reaction wall.

Further, for example, as shown in Non-Patent Document 1 and Non-Patent Document 2, a method of reducing and removing heavy metals in groundwater using metal iron of a permeation reaction wall that improves the decomposition rate of contaminants as a metal reductant, etc. Are also listed.
Japanese Patent Laid-Open No. 05-501520 “Journal of Hazardous Materials”, Vol. 42, 1995, p. 201-212 “Environmental Science and Technology”, Vol. 31, 1997, p. 3348-3357

  However, in the method using the pollutant removal treatment facility, equipment costs such as heavy metal wastewater treatment facilities and high thermal decomposition treatment facilities, long-term maintenance and management costs until completion of purification, and excavation / transportation costs when excavating and transporting, There is a problem that the secondary pollution prevention cost for preventing the contaminated soil from being scattered during excavation and transportation becomes high, and the total cost becomes high.

  In addition, in the method that does not use the pollutant removal treatment facility, when purifying contaminated groundwater using a permeation reaction wall with a metal reductant buried, there is a limit to the treatment capacity of the buried metal, so that the There is a problem that it is necessary to replace a buried metal reductant or to bury a large amount of metal reductant because of its low purification capacity.

The present invention has been made in view of the above points. When purifying contaminated groundwater, the total cost is reduced, the safety is high for a long period of time, the removal performance and permeability are stable and the pollutant is good. It is an object of the present invention to provide a purification method for a contaminated aquifer using a purification agent for water containing pollutants that can be removed.

In order to solve the above problems, the invention according to claim 1 is characterized in that the aquifer containing arsenic as a pollutant contains carbon in an amount of 0.29 wt% or more and 5 wt%, an average particle diameter of 0.1 mm or more, and The arsenic is removed by embedding a purification reagent comprising iron powder having a specific surface area of 5 m ² / g or more.

According to the first aspect of the present invention, cementite Fe ₃ -C is produced on the iron surface by being a purification reaction agent comprising iron powder containing 0.29 wt% to 5 wt% of the carbon. This facilitates the increase in the surface of the cathode, enhances the iron oxidation reaction, and improves the reaction between the iron powder and the contaminants. Further, the by an average particle size of 0.1mm or more and consisting of a specific surface area of 5 m ² / g or more iron powder purification reaction agent, without slowing the permeation rate of the contaminated water, heavy metals contained in the contaminated water The specific surface area in contact with contaminants such as can be increased.

Furthermore, the contaminated water, by those containing contaminants of heavy metals is arsenic, said iron powder and arsenic purification reaction agent can be a good chemical reaction.

Moreover , the aquifer containing the contaminant can be purified by embedding the purification reaction agent.

The invention according to claim 2 contains 0.29 wt% or more and 5 wt% of carbon in the downstream portion of groundwater flowing through an aquifer containing arsenic as a pollutant , has an average particle size of 0.1 mm or more, and a specific surface area A permeation reaction wall is formed by embedding a purification reaction agent made of iron powder of 5 m ² / g or more, and the arsenic is removed by allowing the groundwater to permeate the permeation reaction wall.

According to the second aspect of the present invention, the purification reaction agent is buried in a downstream portion of the groundwater flowing through the aquifer containing the pollutant to form the permeation reaction wall and permeate the contaminated groundwater. Thus, contaminants can be removed.

According to the invention described in claim 1, since the reaction between the metal reductant, which is iron powder containing carbon, and the pollutant can be improved, the effect of improving the pollutant removal performance can be obtained. Play. In addition, by using iron powder having an average particle size of 0.1 mm or more and a specific surface area of 0.5 m ² / g or more, it is possible to maintain the permeation rate of the contaminated water and to secure the specific surface area in contact with the contaminant contained in the contaminated water. As a result, it is possible to maintain the removal rate of the pollutant in the polluted water and to facilitate the reaction between the pollutant and the iron powder.

Moreover , since the iron powder of the purification reagent and the pollutant which is arsenic can react satisfactorily, the contaminated water containing heavy metals can be purified well.

Furthermore, since it can be purified by burying a purification reagent that purifies contaminated groundwater in an aquifer containing pollutants, it does not construct a purification treatment facility, reduces the total cost, and is highly safe for a long time. The removal performance and permeability are stable and the contaminants can be removed, and the purification treatment agent can be buried preventively.

According to the second aspect of the present invention, the purification reaction agent is buried in the downstream portion of the groundwater flowing through the aquifer containing the pollutant to form the permeation reaction wall, and the pollutant is permeated through this. Effectively reducing costs, providing high safety for a long period of time, providing stable removal performance and permeability with good removal of pollutants as well as preventive embedding of purification agents Play.

  Hereinafter, an embodiment of a purification agent for water containing a pollutant according to the present invention will be described. However, it is not limited to the following embodiment.

The water purification reaction agent containing the pollutant in the present embodiment contains 0.29 wt% to 5 wt% of carbon, and has an average particle diameter of 0.1 mm or more and a specific surface area of 5 m ² / g or more. Consists of.

  Iron powder used as a purification reagent is a metal that acts to reduce heavy metal ions and deposit them on its surface. It is oxidized to form colloidal insoluble hydroxides and pollutants such as heavy metals. Coprecipitates. If it is not an iron powder but a metal reductant, it has this action, but iron powder is preferable because it has high reactivity, high safety, low price, and easy handling.

The purification reaction agent is an iron powder containing 0.29 % by weight or more of carbon. By being an iron powder containing carbon, it is easy to produce cementite Fe ₃ C on the surface of the iron powder, thereby increasing the cathode surface and promoting the oxidation reaction of iron. Therefore, when the proportion of carbon in the iron powder increases, the reaction rate is further improved and the removal performance of heavy metals and the like can be enhanced if the specific surface area of the iron powder is large. This is because an iron powder containing less than 0.06% by weight of carbon slows the elution rate of iron and lowers the removal performance of heavy metals and the like, so a large amount of iron powder is required and the cost is increased. In order to further improve the iron extraction rate and the removal performance of heavy metals and the like, iron powder containing 0.3 wt% or more of carbon is preferable.

  The average particle diameter of the iron powder used for the purification reaction agent is an average particle diameter of 0.1 mm or more. This is because, with iron powder having an average particle size of 0.1 mm or less, the permeability of the permeation reaction wall is reduced, and contaminated groundwater is less likely to pass through the permeation reaction wall, thereby reducing the contaminant removal performance. In order to improve the permeability of the contaminated groundwater of the permeation reaction wall, the average particle diameter is preferably 0.25 mm or more. In order to make it the best, an average particle diameter of 0.5 mm or more is preferable.

The specific surface area of the iron powder used for the purification reaction agent is a specific surface area of 5 m ² / g or more. This is because if the specific surface area of the iron powder is small, the specific surface area in contact with contaminants such as heavy metals becomes small and the removal performance is lowered, so that a large amount of iron powder is required and the cost becomes high. A specific surface area of 5.0 m ² / g or more is preferable in order to increase the specific surface area to enhance the removal performance of heavy metals and to ensure a favorable reaction.

  Therefore, the iron powder of the purification reaction agent preferably has a fine powder shape, a slice shape, or a steel wool shape containing the above-described carbon and satisfying the average particle size and specific surface area. Moreover, the metal powder and metal piece which are not contaminated and which generate | occur | produce by a metal processing process may be sufficient.

Contaminants contain arsenic, other organic halogen compounds such as carbon tetrachloride, trichlorethylene, PCB, or one or more heavy metals such as cadmium, lead, cyan, hexavalent chromium, mercury, alkylmercury and selenium. It contains. As shown in the following reaction formula, heavy metals are preferable because they easily generate iron ions of iron powder from the difference in standard electrode potential, and easily react or coprecipitate with water-soluble iron ions. Furthermore, as shown in the following reaction formula, heavy metals that are likely to be present as anions, such as hexavalent chromium, arsenic, cyan, selenium, etc., are preferable because they react well with iron ions.

The operation will be described below.
The water containing the pollutant reacts with the metal reductant containing carbon which is the purification reaction agent to become an insoluble pollutant-metal reductant, and removes the pollutant.
In particular, when the metal reductant is iron, a heavy metal dissolved in water as an anion reacts with the iron ion to form an insoluble matter of iron powder-heavy metal. Or it is taken in and co-precipitated in the iron ion which has formed the colloid, and forms an insoluble matter.

In accordance with the following reaction formula, a mechanism for removing arsenic such as heavy metal by a metal reductant (iron powder) will be described.
According to the following reaction formula, iron powder reacts with dissolved oxygen in water or is ionized to convert Fe ²⁺ and Fe ³⁺ from the difference in standard electrode potential of water-soluble heavy metal M (divalent and trivalent). Produces and dissolves easily in water. Further, the water-soluble heavy metal is transferred to the surface of the iron powder by passing electrons and removed. (Aq: water-soluble, s: insoluble)
2Fe (s) + O ₂ (aq) + 2H ₂ O → Fe ²⁺ (aq) + 4OH ⁻
Fe (s) + M ²⁺ (aq) → Fe ²⁺ + M (s)
4Fe ²⁺ (aq) + O ₂ (aq) + 4H ⁺ → 4Fe ³⁺ (aq) + 2H ₂ O
Fe (s) + M ³⁺ (aq) → Fe ³⁺ + M (s)

As shown in the following reaction formula, this Fe ³⁺ reacts with oxide ions of heavy metal M in water to form iron powder-heavy metal oxide, which becomes insoluble. The water-soluble heavy metal M is taken in as it is and co-precipitated and becomes insoluble when iron hydroxide (divalent or trivalent) is formed in a colloidal form. By becoming insoluble, the heavy metal M is removed from the water. (N, m and p are natural numbers)
Fe ³⁺ (aq) + nMO _m ³⁻ (aq) → Fe (MO _m ) _n (s)
Fe ³⁺ (aq) + M ^{p +} (aq) + (p + 3) OH ⁻
→ M (OH) _p , Fe (OH) ₃ (s)

In particular, arsenic that is an arsenate ion in water becomes insoluble according to the following reaction formula.
Fe ³⁺ (aq) + AsO ₃ ³⁻ (aq) → FeAsO ₃ (s)
_{^{Fe 3+ (aq) + AsO 4}} 3- (aq) → FeAsO 4 (s)

In the case of the above reaction, in the iron powder containing carbon, cementite Fe ₃ —C is easily generated on the iron surface, the cathode surface is further increased, and the oxidation reaction of the iron powder is promoted. In addition, iron powder with a large average particle size undergoes a reaction between iron powder, dissolved oxygen, and heavy metal sequentially without any delay, and iron powder with a large specific surface area has a large surface area in contact with heavy metal and promotes a reaction with heavy metal. To do.

  As described above, the contaminants can be removed from the water containing the contaminants by the iron powder of the purification reagent.

Next, an embodiment of a purification method for a contaminated aquifer using the above-described purification reagent will be described.
Identify the groundwater flow direction, flow velocity, and contamination concentration in soils contaminated with or likely to be contaminated with heavy metals and other pollutants as indicated in the above guidelines. From the results, the permeation reaction wall groove was excavated in the downstream part of the groundwater flowing through the aquifer where contaminated groundwater flows up to the impermeable layer below the aquifer perpendicular to the direction in which the groundwater flows. To a thickness of 300 cm. In this formed groove, 0.06% by weight or more of carbon is contained, the purification reaction agent made of iron powder having an average particle size of 0.1 mm or more and a specific surface area of 0.5 mm ² / g or more is buried, A permeation reaction wall is formed.

  The raw material of the purification reaction agent used for the permeation reaction wall may be mixed with stones, rakes, and the like in order to increase water permeability in addition to the iron powder of the metal reductant. The thickness of the groove of the permeation reaction wall is 30 cm to 300 cm depending on the metal reductant removal performance and the concentration of contaminated groundwater. From the viewpoint of the cost of the metal reductant and securing the burial location, 50 cm to 200 cm is preferable.

  When the permeation reaction wall is formed, the drill & jet method may be used in the case of a loose ground structure made of gravel. In this method, a test digging is first excavated to reach the impermeable layer below the aquifer at an appropriate interval at the position where the permeation reaction wall is formed. Next, pipes that reach the bottom are inserted into each borehole, and the purifying reaction agent is injected between the gravel soil and other materials in the ground from the borehole through the pipe under pressure while removing the pipe. To do. The distance between the boreholes, the amount of iron, and the injection pressure are determined so that the permeation reaction wall is not interrupted and a sufficient thickness is maintained.

  According to the operation of the present invention, ionized substances or the like dissolved from the soil contaminated with the contaminants flow into the groundwater, and the contaminated groundwater passes through the permeation reaction wall in which the purification reaction agent is embedded. According to the above reaction formula, carbon is contained in the purification reagent and a metal reductant (iron ion: divalent, trivalent) ionized by dissolved oxygen in the groundwater, and the contaminated groundwater permeates through the permeation reaction wall. The ionized metal reductant and the ionized contaminant are combined to form an insoluble contaminant-metal reductant (iron), and the contaminant is removed from the groundwater. In particular, in the case of heavy metal arsenic, which is a contaminant, and iron, which is a metal reductant, arsenate ions are formed in groundwater and react with iron ions to form iron arsenate.

  As described above, the groundwater contaminated by the soil contaminated with the contaminant is removed through the permeation reaction wall using the purification agent, and the contaminated groundwater is purified. In particular, groundwater contaminated by spilling from soil contaminated with heavy metal arsenic is purified and becomes safe groundwater that does not contain heavy metals below the national guidelines.

  The present invention will be described in more detail with reference to examples.

[Example 1]
80 mL of simulated arsenic-contaminated groundwater with a concentration of 10 mg / L is placed in a 160 mL vial, and 0.5 g of iron powder 1 (specific surface area 5.6 m ² / g, carbon content 0.29% by weight) is added. The air was replaced with nitrogen, sealed, and shaken for 3 days. After shaking, filtration was performed with a filter having a pore diameter of 0.45 μm, and the concentration of arsenic remaining in the solution was measured.
The arsenic removal rate (%) was defined as (initial concentration before execution−residual concentration after execution / initial concentration before execution) × 100.

  The simulated arsenic-contaminated groundwater in this experiment was prepared by diluting a sodium arsenite aqueous solution (manufactured by Kishida Chemical Co., Ltd.) to a concentration of 10 mg / L using water obtained by treating tap water with activated carbon. Is substituted with nitrogen. The measurement method was the hydride generation ICP emission analysis method (Rigaku ICP emission analysis apparatus).

[Example 2]
The arsenic removal rate of iron powder 2 (specific surface area 1.5 m ² / g, carbon content 0.29 wt%) was determined in the same manner as in Example 1.

[Example 3]
The arsenic removal rate of the iron powder 3 (specific surface area 4.7 m ² / g, carbon content 0.06 wt%) was determined in the same manner as in Example 1.

[Example 4]
The arsenic removal rate of iron powder 4 (specific surface area 0.1 m ² / g, carbon content 1 wt%) was determined in the same manner as in Example 1.

The results of Examples 1 to 4 are shown in Table 1.

  From Table 1, the carbon content was the same as in Example 1 and Example 2, but the arsenic removal rate increased by 17% due to the increase in specific surface area. Moreover, from Example 3, the iron powder containing 0.06% of carbon was able to remove 20% of arsenic. Furthermore, from Example 1 and Example 3, although the specific surface area of the iron powder was substantially the same, the arsenic removal rate increased by 62% when the proportion of carbon contained in the iron powder increased by 0.23% by weight. Moreover, even if the specific surface area of iron powder was small from Example 2 and Example 3, when the ratio containing the carbon in iron powder increased, the arsenic removal rate increased by 35%. Further, from Example 4, the iron powder containing 1% by weight of carbon was able to remove 22% of arsenic.

From the above, it was clarified that the iron powder increases the arsenic removal rate as the specific surface area increases .

Furthermore, from Example 1 to Example 4, the iron powder containing carbon tends to generate cementite on the surface of the iron powder in the iron powder, the cathode surface is increased, the oxidation reaction of the iron powder is promoted, and arsenic It was clarified that the removal rate increased dramatically. Moreover, it turns out that the arsenic removal rate becomes 82% by using iron powder containing 0.29% of carbon from Example 1, and the arsenic removal rate increases as the carbon content increases.

Further, the amount of iron powder corresponding to heavy metal as a purification reaction agent is 0.3% by weight of carbon from Example 1, and when the iron powder has a specific surface area of 5.6 m ² / g, the removal rate is 80%. In the above case, 0.49 g or more of iron powder was preferable with respect to 1 mg of heavy metal to be processed, and when the removal rate was 100%, it was considered that 0.61 g or more of iron powder was preferable with respect to 1 mg of heavy metal to be processed.

[Example 5]
The water permeability coefficient of iron powder 5 (average particle size 0.52 mm) was measured by a constant water level method. The measuring instrument used was a soil permeability measuring instrument DIK4,000 (manufactured by Daiki Rika Kogyo Co., Ltd.).

[Example 6]
The water permeability coefficient of iron powder 6 (average particle size 0.45 mm) was measured by the same water level method as in Example 5.

[Example 7]
The water permeability coefficient of iron powder 7 (average particle size 0.09 mm) was measured by the same water level method as in Example 5.

[Example 8]
The water permeability coefficient of iron powder 8 (average particle size 0.04 mm) was measured by the same water level method as in Example 5.

The results of Example 5 to Example 8 are shown in Table 2.

From Table 2, from Example 5 to Example 8, the permeability coefficient is also increased by increasing the average particle diameter. In general, the permeability coefficient of the aquifer is from 0.1 m / day to 10 ² m / day, and the permeability of the permeation reaction wall is less than 0.1 m / day, so it is difficult for groundwater to pass through. Decreases, and the processing speed decreases. Therefore, when the average particle size is 0.09 mm or less, it becomes 0.1 m / day or less, and it has been clarified that the removal performance is greatly deteriorated.

From the above, the arsenic removal performance is high due to the iron powder containing 0.29 wt% to 5 wt% of carbon, having an average particle size of 0.1 mm or more and a specific surface area of 5 m ² / g or more. It was revealed that the nature is also high.

Claims (2)

A purification reagent comprising iron powder containing 0.29 wt% or more and 5 wt% carbon in an aquifer containing arsenic as a pollutant , having an average particle size of 0.1 mm or more and a specific surface area of 5 m ² / g or more. A method for purifying a contaminated aquifer, wherein the arsenic is removed by burying. Iron powder containing carbon in the downstream portion of groundwater flowing through an aquifer containing arsenic as a pollutant and having an average particle size of 0.1 mm or more and a specific surface area of 5 m ² / g or more. A method for purifying a contaminated aquifer, comprising forming a permeation reaction wall by embedding a purification reaction agent comprising, and removing the arsenic by allowing the groundwater to permeate the permeation reaction wall.