JP2007289897A - Purification method of polluted soil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the purification method of polluted soil for surely removing harmful substances from the polluted soil with little environmental load since natural energy is used, at a low cost, and over a long period of time span. <P>SOLUTION: This purification method of polluted soil comprises: a first process for burying at least a pair of electrodes into the surface layer of the high moisture-content polluted soil with a given space, applying a current between the electrodes and moving the harmful substances eluted into pore water in the polluted soil electrochemically toward the electrodes for accumulation; and a second process for cultivating harmful substance highly-accumulating plants in the surface layer of the polluted soil, making the plants absorb the harmful substances, and cropping the plants after absorption of the harmful substances. A third process can be included for burning the harmful substance highly-accumulating plants cropped in the second process as a biomass fuel to generate heat and electric power, and separating the harmful substances from the fuel in the form of combustion ash. The generated power can be supplied to the electrodes in the first process. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for purifying highly water-contaminated soil containing harmful substances, and more particularly to a method for purifying contaminated soil with a small environmental load by combining an electrochemical method and a biological method.

  Conventionally, industrial wastes such as coal ash and various combustion ash generated from industrial activities contain hazardous substances such as heavy metals, so they are landfilled at a management-type disposal site, and the landfill disposal site is effective. It was not used. However, in recent years, the effective use of such disposal sites has been studied. In order to make effective use of the disposal site, it is assumed that the soil made of coal ash is not contaminated with harmful substances, and soil contaminated with harmful substances (contaminated soil) is inherent in accordance with the “Soil Environmental Standards”. It is necessary to purify harmful substances. According to the standard, volatile organic compounds, heavy metals, agricultural chemicals and the like are listed as harmful substances.

  The techniques for remediating soil contaminated with toxic substances are roughly divided into in-situ purification and excavation and removal methods, and the former is further classified into in-situ extraction and in-situ decomposition methods. The in-situ extraction method is a method for purifying contaminated soil by removing harmful substances in the contaminated soil extracted by means such as suction of soil gas in the contaminated soil or pumping of groundwater. According to this technology, in addition to the suction equipment and pumping equipment for soil gas, equipment for decomposing and separating the extracted gas and toxic substances in the groundwater is necessary, so that there is a problem that equipment costs and operation costs are high. .

  The in situ decomposition method is a method for purifying the contaminated soil by chemically or biologically decomposing harmful substances in the contaminated soil in situ. This method is advantageous in terms of cost compared to the in-situ extraction method, but has a drawback that it cannot be applied to heavy metals other than cyan.

  The excavation and removal method is a method in which contaminated soil is removed by excavation, and the removed soil is usually subjected to heat treatment or washing treatment. However, this technology requires equipment for soil excavation and subsequent transportation, which is costly and requires heating equipment and washing equipment for the removed soil. There is a problem that it is required and the environmental load is very large.

  As is currently the case, the management-type disposal site is set up on a vast site and the contaminated area is extensive, and the contaminated soil is purified by the above in-situ extraction method and excavation method. Removing material requires enormous costs and may exceed the value of the area (land). For this reason, development of an inexpensive purification technique is desired, but considering the current situation, it is desired that the purification technique has a small environmental load.

  On the other hand, techniques for electrochemically removing harmful substances from contaminated soil in situ have been proposed recently (see Patent Documents 1 to 3). The technologies related to these proposals are based on electroosmosis and electrophoresis in which harmful substances in contaminated soil move toward the negative electrode through the eluent by applying a voltage between at least a pair of positive and negative electrodes embedded in the soil. It uses the electrokinetic phenomenon such as. Compared with the excavation and removal method described above, this technology has advantages such as less risk of exposure to workers, little diffusion of contaminants outside the treatment area, and application to non-permeable grounds. Have. However, each of the above patent documents discloses a technique mainly aimed at eluting harmful substances from contaminated soil and separating them in the state of the eluate, and requires processing equipment for the separated eluate. There is a problem that the cost for that is also required.

  In addition, research on an environmental restoration technique called so-called phytoremediation is in progress (for example, see Patent Document 4). This technology cultivates plants with a high accumulation of harmful substances using contaminated soil containing harmful substances as a soil, and absorbs the harmful substances in the soil by utilizing the ability of the plants to absorb moisture and nutrients from the roots. In this way, harmful substances are removed from contaminated soil. For example, cadmium removal by rice, arsenic removal by fern, dioxins removal by tobacco and potato, etc. have been studied and commercialized. Although this technology is superior in that it has a very low environmental impact, at present, the above research focuses on identifying plants that can accumulate harmful substances at high concentrations and harmful substances that can be applied to these plants. However, the current situation is that sufficient studies have not been made on the treatment of plants that have accumulated harmful substances from the viewpoint of recycling systems that can save resources and reduce the environmental burden.

JP-A-5-59716 JP-A-8-281247 JP 2003-31256 A JP 2005-199209 A

  In view of the above circumstances, the present invention provides a method for purifying contaminated soil that uses natural energy, has a low environmental load, is low in cost, and can reliably remove harmful substances from contaminated soil over a long span. For the purpose.

  As a result of intensive studies based on the above-mentioned purpose, the present inventor combined a conventionally known electrochemical method and phytoremediation technology to accumulate harmful substances in the contaminated soil in the plant, and the high concentration By harvesting plants that have accumulated harmful substances, it is possible to purify contaminated soil, and as a result, we have acquired knowledge that a method for purifying contaminated soil with a low environmental load using natural energy can be established over a long period of time. The present invention has been completed.

  That is, the present invention embeds at least one pair of electrodes in a surface layer of contaminated soil having a high water content at a predetermined interval, and electrochemically removes harmful substances eluted in pore water in the contaminated soil by energizing the electrodes. The first step of moving toward the electrode and accumulating it, and cultivating a harmful substance highly accumulating plant on the surface layer of the contaminated soil, absorbing the harmful substance into the plant, and absorbing the harmful substance The above problem is solved by a method for purifying contaminated soil, comprising a second step of harvesting plants.

  In the present invention, the harmful substance high-accumulation plant harvested in the second step is burned as biomass fuel to generate heat and electric power, and the harmful substance is separated from the fuel in the form of combustion ash. A process may be included. The generated power can be supplied to the at least one set of electrodes in the first step. This combustion discharges carbon dioxide absorbed by the plant into the atmosphere, so the amount of carbon dioxide in the atmosphere is not increased, and the resulting power can be used effectively in the first step, further increasing the environmental load. Can be reduced.

  According to the present invention, it is possible to purify contaminated soil containing hazardous substances in situ by combining electrochemical treatment and biological treatment. It can be used effectively and the environmental load can be reduced.

  Examples of soil contaminated with harmful substances that are the subject of the present invention include, for example, agricultural land, industrial land, urban areas, residential areas, etc. Examples include soil landfilled at a waste disposal site. In particular, when the contaminated soil is coal ash containing heavy metals landfilled at a managed disposal site, the coal ash is excellent in water absorption and water retention, so it is suitable as a plant growth environment. The purification method of the invention can be suitably carried out.

  Hazardous substances that can be purified in the contaminated soil purification method of the present invention include inorganic compounds and organic compounds. Specific examples of the former include metals (heavy metals) such as boron, cadmium, cobalt, chromium, copper, mercury, nickel, lead, and selenium; radioactive materials such as cesium, uranium, deuterium, and strontium; arsenic, chromate, Examples include fluorine, sodium, and ammonia. Specific examples of the latter include chlorinated substances such as TCE, PCE, PCB, pyrene and MTBE; explosive substances such as DNT, TNT and RDX; petroleum-based organic compounds such as BTEX and TPH; wood such as PCP and PAHs Preservatives: Pesticides such as atrazine, bedazone, DDT, 2,4-T, insecticides and the like. Among these, the purification method of the present invention can be suitably used for the purification of heavy metals.

  Hereinafter, with reference to FIG. 1, the purification method of the contaminated soil of this invention is demonstrated in detail. FIG. 1 shows a schematic flow diagram in the case where the coal ash 1 generated in a thermal power plant and disposed of in landfill is used as contaminated soil, and the method for purifying contaminated soil of the present invention is applied to the disposal site 2.

  As shown in FIG. 1, the contaminated soil purification method of the present invention comprises three steps. The first step is a step of accumulating heavy metals in the coal ash 1 in the disposal site 2 on the surface layer using an electrochemical technique, and between at least one pair of electrodes embedded in the coal ash 1 at a predetermined interval. In the past, a so-called electroosmotic action, electrophoretic action, or electromigration action that moves harmful substances contained in the effluent in the soil existing between the electrodes to the electrodes in a liquid phase state by applying current to the electrodes. A known electrochemical treatment is performed.

  In the first step, heavy metals in the coal ash 1 are moved toward the electrodes by using an eluent present in the gaps of the coal ash 1 as a medium, and the coal ash 1 is energized between the electrodes embedded therein. Therefore, it is necessary that the water content is at least high enough to ensure electrical conduction. Preferably, the coal ash 1 has a saturated zone close to the ground surface and has a water content close to or higher than the saturated water content. When the coal ash 1 is dry and electrical conduction cannot be ensured, it is necessary to supply water from the surface of the coal ash 1. As the water supply, normal tap water, industrial water or the like can be used, but an aqueous solution such as an acid, an electrolyte or a chelating agent can also be used in consideration of properties such as soil pH. Water supply can be performed by, for example, a method of spraying water directly from the surface of the coal ash 1.

  The shape of the electrode used in the first step is not particularly limited, and can be appropriately selected from, for example, a plate shape, a rod shape, or a tubular shape. The length of the electrode is not particularly limited as long as at least the tip reaches the saturation zone in the coal ash 1, but is longer than the rhizosphere of the plant cultivated on the surface layer of the coal ash 1 in the second step described later. It is preferable that the length is such that the tip reaches a deep position.

  Usually, at least one set of electrodes is vertically embedded in the coal ash 1, but the electrode is not limited to this direction, and can be embedded, for example, at a predetermined angle from the vertical direction. When embedding a plurality of pairs of electrodes in the surface layer of the coal ash 1, the electrodes can be regularly arranged, for example, arranged on the same straight line. Note that the above “one set” means not only a case where there are only one anode and a cathode, but also a case where a plurality of cathodes are arranged for one anode, and one electrode for a plurality of anodes. It is used in the meaning including the case of arranging the cathode.

  A voltage is applied between the electrodes arranged in this way, and electricity is applied between the anode and the cathode. The voltage value and current value at that time can be appropriately changed depending on the harmful substance to be removed. Usually, when the harmful substance is a heavy metal such as hexavalent chromium, it is about 40 to 60 V and 6 to 3 A according to the electrophoretic action, and about 50 V and 10 to 20 A according to the electroosmotic action. In the case of lead or arsenic, a current of about 0.1 A is set to flow between the electrodes according to the electromigration action. The voltage may be either direct current or alternating current. When electricity is applied between the electrodes, an electroosmosis phenomenon occurs in which moisture in the soil moves from the anode side to the cathode side by electroosmosis. This phenomenon can cause the moisture content in the coal ash 1 near the anode to decrease, but if the moisture content decreases, the moving speed of heavy metals decreases and the removal capability decreases. Accordingly, it is preferable to supply water to the surface layer on the anode side. In addition, when the pH of the coal ash 1 between the electrodes is in the neutral range or higher, it is possible to expect the effect of sprinkling a pH adjuster solution or the like to lower the soil pH and promoting ionization of the hardly soluble heavy metal. .

  In the second step, in the surface layer of coal ash 1 in which harmful substances are accumulated on the surface layer in the first step, a plant with a high accumulation of harmful substances is cultivated using this as soil, and the plant absorbs the harmful substances in the coal ash 1 Reduce the concentration of harmful substances. Thereby, it is possible to remove harmful substances from the coal ash 1.

  The harmful substance high-accumulation plant used in the present invention is a plant that can accumulate and transport a large amount of harmful substances in the plant body and is resistant to toxicity. Preferably, it is a plant that grows quickly, spreads the roots over a wide area, moves harmful substances absorbed therefrom to the foliage part, and accumulates in the part. Plants with a high accumulation of harmful substances can be used according to the type of harmful substances.

  When the harmful substance is a heavy metal, it has been found that about 400 kinds of plants have an accumulation ability as a result of previous studies. For example, there are varieties such as Brassicaceae, Convolvulaceae, Sweet potato, Legume, Macadamia, and Rubiaceae. well known. In the case where the harmful substance is a radioactive substance, examples include cruciferous, red-breasted plants, willows, sunflowers, corn, and related species that have been studied recently and have been shown to absorb and accumulate radioactive substances. When the harmful substance is an exogenous organic pollutant such as agricultural chemicals, TCE, petroleum, TNT, PAHs, PCP, PCBs, etc., rice having a decomposing action, cyperaceae and the like can be mentioned. For corn and clover for petroleum (PAH), Italian ryegrass, fepot and fescue for diesel oil and crude oil, perennial ryegrass for wood preservatives, pyreneaceae for PCB and TNT, Examples include grass and leguminous plants, meadow bromegrass, petroleum-based compounds such as bamu douglas, and diesel oil such as perennial ryegrass. These can be used alone or in combination of two or more depending on the type of harmful substance to be removed from the contaminated soil.

  Moreover, in this invention, the plant which absorbed the harmful | toxic substance and recombined the gene so that it might have tolerance with respect to the harmful | toxic substance can also be used suitably as a harmful substance high accumulation | storage plant. For example, a predetermined gene is isolated from the above-exemplified plants, and the same or different plants incorporating the isolated gene can be used.

  The plant may be sown on the surface layer of coal ash 1 at an appropriate time and grown as it is, or a seedling or a plant body grown in advance may be transplanted to the surface layer. Moreover, you may use the plant which inhabits this landfill disposal site from before. Seeding and transplantation can be performed by a conventionally known method.

  In this invention, you may use together the substance which assists absorption of a harmful | toxic substance with the harmful substance high accumulation plant. For example, a clay mineral such as zeolite can be suitably used. In addition, coal ash and processed products thereof, particularly granulated products, are extremely excellent in water absorption and water retention, and can take in harmful substances together with the elution water, so these may be used for this purpose. These substances that assist in the absorption of harmful substances are distributed or buried in a known manner on the surface of the disposal site 2 (particularly around the electrode) or in the ground.

  Said 1st process and 2nd process may be implemented in this order, and can also be performed simultaneously in parallel. In the latter case, at least one pair of electrodes can be embedded in the surface layer of the landfill disposal site according to the above-described electrode arrangement, and seeds of highly harmful substance-accumulating plants can be cultivated on the surface layer. In addition, the landfill disposal site is divided into, for example, two sections, the first process is performed in one section, the second process is performed in the other section, and after the grown plants are harvested, the second process is performed in the one section. On the other hand, the first step may be performed. The ruins may be further divided into a plurality of sections, and the first process and the second process may be alternately performed in each section.

  After a predetermined period of time, plants that have absorbed harmful substances are harvested and then dried. The method for harvesting and drying is not particularly limited, and a conventionally known method can be appropriately selected. This makes it possible to remove the harmful substances contained in the highly water-contaminated soil and purify the contaminated soil. The purified soil and the landfill of the soil can be effectively used.

  This process is low-cost and at the same time uses solar energy and does not increase carbon dioxide, so there is almost no environmental impact and is effective in preventing the diffusion of contaminated soil, greening, and the like. In particular, it can be applied to low-concentration and wide-ranging soil pollution purification, and the application range is very wide including organic, inorganic and radioactive pollutants.

  In the third step, the plant harvested and dried in the second step is subjected to processing such as pulverization if necessary, and then this is used as fuel (hereinafter referred to as fuel thus processed). Biomass power generation is performed as “biomass fuel”), and electricity (described as generated power in FIG. 1) and heat are obtained. The crushing equipment used is not particularly limited, and an equipment capable of efficient crushing treatment can be selected in consideration of the form of the plant. Further, if necessary, the dried plant can be subjected to a treatment such as forming into a lump shape by pressure molding or the like.

  Examples of the biomass fuel combustion facility include a boiler capable of directly combusting it. When burning this biomass fuel in a coal fired boiler, it may be co-fired with the coal fuel. The resulting heat and steam can be used for other applications. In this way, when the biomass fuel is burned and disposed using a directly combustible boiler, the carbon dioxide absorbed by the photosynthesis by the plant is only discharged into the atmosphere, so the amount of carbon dioxide in the atmosphere is not increased. . Further, when biomass fuel is co-fired with coal fuel, the amount of coal fuel used can be reduced by the amount of biomass fuel mixed, so that the increase in the amount of carbon dioxide in the atmosphere can be suppressed.

  The obtained generated power 10 can be transmitted to the power system. In addition, power can be supplied to at least one set of electrodes in the first step by appropriately converting to direct current and storing the power (see reference numeral 11 in FIG. 1). In the latter case, power supply can be contributed to leveling of power demand by supplying power in the late-night time when power demand is reduced.

  When the harmful substance in the biomass fuel is, for example, heavy metal, the heavy metal contained in the combustion ash generated by the combustion of the fuel can be separated and extracted or insolubilized for disposal. Since harmful substances are contained at high concentrations in the combustion ash, which is a very small amount than the amount of this contaminated soil, it is possible to reduce the concentration.

It is a flowchart which shows an example of embodiment of the purification method of the to-be-processed soil of this invention.

Explanation of symbols

1 Coal ash 2 Unused ruins of coal ash disposal site 3 1st process (electrochemical treatment process)
4 Second process (phytoremediation process)
5 Second step (plant harvesting)
6 Third process (biomass power generation process)
8 Incineration ash

Claims (6)

  At least one set of electrodes is embedded at a predetermined interval on the surface layer of highly water-contaminated soil, and harmful substances eluted in the pore water in the contaminated soil are electrochemically directed to the electrodes by energizing the electrodes. A second step of cultivating a plant with a high accumulation of harmful substances on the surface layer of the contaminated soil, absorbing the harmful substances into the plant, and harvesting the plants after absorbing the harmful substances. A method for purifying contaminated soil, comprising a step.   The method for purifying contaminated soil according to claim 1, further comprising a third step of generating electric power and heat by burning the harvested plant as biomass fuel.   The method for purifying contaminated soil according to claim 1 or 2, wherein the electric power is supplied to at least one set of electrodes in the first step.   The method for purifying contaminated soil according to any one of claims 1 to 3, wherein, in the second step, a substance having an ability to absorb harmful substances is used in combination with the harmful substance high accumulation plant.   The method for purifying contaminated soil according to any one of claims 1 to 4, wherein the contaminated soil is accumulated in a landfill site. The method for purifying contaminated soil according to any one of claims 1 to 5, wherein the harmful substance is a heavy metal.

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