JP5069056B2 - Purification method for heavy metal contaminated soil - Google Patents

Description

  The present invention relates to a method for efficiently and inexpensively removing heavy metal elements contained in biomass such as rice straw, rice husks, soybeans, and plants, and a method for purifying contaminated soil containing heavy metal elements. More preferably, the present invention relates to a method for producing high-purity amorphous silica from biomass after removal of heavy metal elements.

  It is clear that heavy metal elements such as lead, cadmium, and arsenic have adverse effects on the environment and the human body. Among them, cadmium is mixed into the soil from abandoned mines and old smelters, and forms highly contaminated soil in the suburbs of the abandoned mines and downstream paddy fields. Daily intake of brown rice cultivated in such contaminated paddy fields is the main cause of itai-itai disease.

  In the Food Sanitation Law, the allowable cadmium concentration in brown rice is set to 1 ppm or less. However, in recent years, stricter international standards for cadmium concentrations in foods have been studied by the World Health Organization (WHO), and consumer interest in food safety has increased. Therefore, it is necessary to further reduce the cadmium concentration in agricultural products.

  When there is a possibility that the cadmium content in brown rice may exceed the new standard value, a method (customer soil) of filling uncontaminated soil in paddy fields may be performed. However, since this land cost is very high, it is difficult to continuously put it into practical use in terms of economy. Thus, several methods have been proposed in the past as measures for removing heavy metal elements in paddy fields of contaminated soil containing heavy metal elements.

  First, there is a contaminated soil restoration technique (phytoremediation) using plants. Certain plants, such as rice, soybean, and rape, can absorb high concentrations of heavy metal elements and have a resistance mechanism against heavy metals, and are called super-accumulation plants. Such plants are cultivated in contaminated soil areas where lead, cadmium, zinc, arsenic, etc. are present, these heavy metal elements are taken into the plant, and the plant is harvested and then incinerated, thereby starting with cadmium Recover heavy metal elements from soil. However, it has been pointed out that this method is likely to cause environmental problems such as the release of heavy metal elements such as cadmium into the atmosphere by incineration.

  On the other hand, the amount of cadmium in soil that is absorbed by plants and crops varies greatly depending on its chemical state. If the soil is reduced to an oxygen-deficient state, cadmium binds to the anion in the soil and becomes difficult to be absorbed from the roots of the plant, and the cadmium concentration in the brown rice can be kept low. Specifically, a method of completely flooding the paddy field is taken 3 weeks before and after the heading period.

In addition, in the “method for purifying heavy metal-contaminated soil” disclosed in Japanese Patent Application Laid-Open No. 2005-169391, when ferric chloride is added to paddy soil containing cadmium, the hydrogen ions and chloride ions that are generated are added to the soil. It is taught that adsorbed cadmium can be extracted efficiently. In this method, iron ions and chlorine ions are generated in paddy water, and the chloride ions react with heavy metal elements such as cadmium to form chlorides, but excess chlorine ions are absorbed by rice straw. There is also a problem.
Japanese Patent Application Laid-Open No. 2005-163981

  An object of the present invention is to provide a method for extracting a heavy metal element contained in biomass such as rice straw, rice husk, soybean, and plant at low cost and with high efficiency.

  Another object of the present invention is to make it possible to put to practical use a contaminated soil repair technique (phytoremediation) by plant cultivation in contaminated soil.

  Still another object of the present invention is to produce high purity amorphous silica from biomass from which heavy metal elements have been removed.

  The method for removing heavy metal elements in biomass according to the present invention includes a step of immersing biomass containing heavy metal elements in a carboxylic acid aqueous solution, a step of eluting heavy metal elements by holding the biomass in a carboxylic acid aqueous solution, And a step of removing the biomass after removal of the heavy metal element from the carboxylic acid aqueous solution.

  Preferably, the carboxylic acid has a hydroxyl group. Alternatively, examples of the carboxylic acid include citric acid, isocitric acid, oxalic acid, tartaric acid, malic acid, and lactic acid.

  Preferably, the temperature of the carboxylic acid aqueous solution is 50 ° C. or higher and 90 ° C. or lower. Preferably, the concentration of the carboxylic acid aqueous solution is 1% or more and 20% or less. Furthermore, preferably, the retention time of the biomass in the carboxylic acid aqueous solution is 1 minute or more.

  Preferably, the biomass is any one of rice straw, rice husk, wheat straw, wheat husk and soybean. In one embodiment, the content of the heavy metal element in the biomass before being immersed in the carboxylic acid aqueous solution is 20 ppm or more, and the content of the heavy metal element in the biomass after the treatment with the carboxylic acid aqueous solution is 5 ppm or less. In another embodiment, the content of heavy metal elements in the biomass after treatment with the carboxylic acid aqueous solution is 1 ppm or less.

  The method for purifying heavy metal-contaminated soil according to the present invention includes growing a plant in contaminated soil containing heavy metal elements, absorbing the heavy metal elements in the plant, and separating the plants that have absorbed the heavy metal elements from the soil. A part or all of the plant separated from the soil as biomass, immersed in an aqueous carboxylic acid solution to elute the heavy metal element into the aqueous solution, and a step of removing the heavy metal element removed biomass from the aqueous carboxylic acid solution .

  In addition to the above steps, the purification method according to one embodiment further includes a step of washing the biomass taken out from the carboxylic acid aqueous solution and a step of heating the washed biomass in an air atmosphere to produce silica. Prepare. Furthermore, this purification method may include a step of spraying silica onto the contaminated soil.

  Plants that grow on heavy metal contaminated soil are, for example, rice or wheat. Preferably, the carboxylic acid has a hydroxyl group. Alternatively, the carboxylic acid is, for example, citric acid, isocitric acid, oxalic acid, tartaric acid, malic acid or lactic acid.

  Preferably, the temperature of the carboxylic acid aqueous solution is 50 ° C. or more and 90 ° C. or less, the concentration of the carboxylic acid aqueous solution is 1% or more and 20% or less, and the retention time of the biomass in the carboxylic acid aqueous solution is 1 minute or more. is there.

  When the washed biomass is heated in the air atmosphere to produce silica, the heating temperature in the air atmosphere is preferably 300 ° C. or more and 1100 ° C. or less, and the produced silica is amorphous silica. is there.

  The technical significance and operational effects of the above configuration will be described in the following items.

  In this invention, the method of extracting a heavy metal element simply and cheaply from biomass containing heavy metal elements, such as cadmium, lead, and arsenic, is proposed. Specifically, a carboxylic acid aqueous solution that is safe and safe for the environment and the human body is managed under appropriate conditions, and the above biomass is immersed in the aqueous solution to utilize a chelate reaction by a carboxyl group. Heavy metal elements contained in biomass are extracted into an aqueous solution. Then, only the heavy metal component is removed by filtering the aqueous solution. According to this method, it becomes possible to treat as hazardous waste without releasing into the atmosphere by incineration. In addition, rice straw and rice husks that have been immersed in a carboxylic acid aqueous solution to remove heavy metal elements are then washed with water and then fired in an appropriate temperature range to produce high-purity amorphous silica. It is also possible.

  The inventors of the present application cultivated plants such as rice, wheat and soybeans in soil contaminated with heavy metal elements such as cadmium, and collected the plants in a state where the heavy metal element components were aggregated in these plants by phytoremediation. To do. Subsequently, the above-mentioned biomass containing the heavy metal element is subjected to an acid cleaning treatment in which the biomass is immersed in a specific carboxylic acid aqueous solution and stirred. By this acid cleaning treatment, heavy metal elements are discharged and removed from the biomass by a chelate reaction with carboxyl groups, and the heavy metal element compounds that settle in the aqueous solution are recovered. According to this method, it is possible to efficiently reduce and recover heavy metal element components contained in paddy soil while suppressing the load on the environment and the human body. In order to realize this, it is necessary to appropriately manage each condition relating to the acid cleaning treatment described below.

(1) Process for preparing biomass containing heavy metal elements By conventional phytoremediation, plants such as rice, wheat, and soybeans are cultivated in paddy fields or nearby soils, and cadmium is included in these plants. Aggregate and accumulate heavy metal elements.

(2) Step of immersing and washing biomass in carboxylic acid aqueous solution The biomass containing a heavy metal element is retained in the carboxylic acid aqueous solution and the heavy metal element is eluted in the aqueous solution. As the carboxylic acid, those having a hydroxyl group are preferred. The purpose of using a carboxylic acid having a hydroxyl group is to capture heavy metal element component ions such as cadmium, lead, and arsenic contained in biomass using a chelate reaction by a carboxyl group constituting the carboxylic acid, and from outside the system in an aqueous acid solution It is for discharging. In order to realize this, any of citric acid, isocitric acid, tartaric acid, malic acid, and lactic acid is desirable as the carboxylic acid. Although oxalic acid does not have a hydroxyl group, it can be used as a preferred carboxylic acid.

  Rice husk and rice straw contain more silica components than other crops and wood, so after extracting the above heavy metal elements, the residue is baked in the air to produce high-purity amorphous silica. Obtainable. In order to realize this, it is preferable to use a carboxylic acid having a hydroxyl group and having 3 or more carboxyl groups, specifically citric acid or isocitric acid, as the carboxylic acid. With such a carboxylic acid, an alkali metal impurity element can be removed in addition to a heavy metal element, and 98% or more of amorphous silica can be obtained as a sample after firing. From the above, when rice straw or rice husk is targeted, it is more desirable to use an aqueous solution of citric acid or isocitric acid.

  The concentration of the carboxylic acid aqueous solution having a hydroxyl group is preferably 1% or more and 20% or less. If the concentration is less than 1%, a sufficient chelating effect for discharging heavy metal element components from the inside of the biomass to the outside of the system cannot be obtained. On the other hand, even if the concentration exceeds 20%, the chelating effect is not improved. On the contrary, if the concentration is too high, there is an economic problem that the number of water washing increases in the water washing process of the residue, which is the step before firing, when silica is produced from the residue in rice straw or rice husk. Will arise.

  About the temperature of the carboxylic acid aqueous solution which has a hydroxyl group, normal temperature or more is preferable, More preferably, it is 50 to 90 degreeC. By increasing the temperature of the aqueous solution to 50 to 90 ° C., more cadmium contained in rice straw or rice husk can be discharged out of the system. Even if the temperature of the aqueous solution exceeds 90 ° C., the effect is not improved, and in the boiling state near 100 ° C., there arises a problem that the water is evaporated and the concentration of the carboxylic acid aqueous solution is changed.

  In order to sufficiently remove the heavy metal element component from the biomass immersed in the carboxylic acid aqueous solution, it is preferable that the retention time of the biomass in the aqueous solution is 1 minute or longer.

  In addition, in the “method for purifying heavy metal-contaminated soil” disclosed in Japanese Patent Application Laid-Open No. 2005-169381, which is a prior art document, the chelating solution treatment is used. Here, paddy soil is washed with hydroxide ions. The cadmium-containing waste liquid discharged in this way has been proposed to be precipitated as a hydroxide by chelating, and the chelating treatment is not intended to extract cadmium from biomass such as rice straw and rice husks. Absent. The chelating solution used here is a pyrrolidine-based, imine-based, carbamic acid-based liquid heavy metal scavenger, which is completely different from the carboxylic acid aqueous solution having a hydroxyl group proposed by the present inventors. Is different.

  In the method of the present invention, the biomass to be washed with the carboxylic acid aqueous solution is preferably rice straw, rice husk, straw, wheat husk or soybean. There is no particular restriction on the content of heavy metal elements in the biomass before being immersed in the carboxylic acid aqueous solution, but if the target is biomass that has sufficiently absorbed and concentrated heavy metal element components in the soil, The heavy metal element content in the previous biomass is, for example, 20 ppm or more, and the heavy metal element content in the biomass after the treatment with the carboxylic acid aqueous solution is, for example, 5 ppm or less. The content of the heavy metal element after the treatment is preferably as small as possible, for example, 1 ppm or less.

(3) Production of high-purity silica by washing the biomass after washing and calcination in the atmosphere Discharge / removal of heavy metal element ions in the biomass from the biomass by chelating reaction with carboxyl groups in the carboxylic acid aqueous solution described above In order to carry out the process more efficiently, it is effective to carry out the washing treatment of the residue after the washing treatment in the carboxylic acid aqueous solution. Although the water-washing treatment at room temperature has a removal effect, the water-washing treatment at a temperature of 50 ° C. or higher further improves the discharge / removal effect. In addition, regarding the water washing treatment conditions, a sufficient removal effect can be obtained by using 20 times or more, more preferably 50 times or more of water with respect to the biomass volume.

  Biomass containing a large amount of silica components such as rice straw and rice husk, which has been immersed and washed in a carboxylic acid aqueous solution, followed by water washing treatment, can be made into a high-purity and amorphous structure by firing in the atmosphere. The silica which has can be obtained. The inventors of the present invention set the temperature to be 300 ° C. or higher and 1100 ° C. or lower in an air atmosphere as a condition for burning rice straw or rice husk that has been subjected to washing treatment and water washing treatment in a carboxylic acid aqueous solution that satisfies the above-described appropriate conditions. Found it desirable. If it is less than 300 degreeC, since a carbohydrate does not fully burn, a residual carbon component will increase and silica purity will fall. On the other hand, when the combustion temperature exceeds 1100 ° C., the amorphous structure becomes a crystalline structure.

  FIG. 1 shows the results of differential calorimetry when three types of rice husks are heated from room temperature to 1000 ° C. The three types of rice husks prepared were immersed in a raw rice husk that had not been cleaned with acid treatment for 15 minutes in a 5% strength aqueous sulfuric acid solution and immersed in a 5% strength aqueous citric acid solution for 15 minutes. Washed rice husk.

  As is clear from FIG. 1, an exothermic reaction is confirmed at around 450 ° C. to 550 ° C. in the raw rice husk. This means burning of carbohydrates (cellulose, hemicellulose, etc.) contained in rice husk. On the other hand, in the rice husk washed with a sulfuric acid aqueous solution and a citric acid aqueous solution, the above exothermic reaction was not confirmed, and on the other hand, an endothermic reaction at 300 ° C. to 400 ° C. was confirmed.

  The above three kinds of rice husk samples were analyzed for decomposition behavior at 400 ° C. by performing pyrolysis gas chromatography mass spectrometry. The result is shown in FIG. In rice husk washed with a sulfuric acid aqueous solution and a citric acid aqueous solution, a peak is detected at a position indicated by a circle in the figure, which means the formation of levoglucosan. That is, by immersing rice husk in sulfuric acid or citric acid aqueous solution, hydrolysis by these acids occurred and the molecular structure of cellulose, which is a polymer contained in rice husk, was changed. On the other hand, the production of levoglucosan has not been confirmed in raw rice husks that have not been subjected to acid treatment cleaning.

  The above hydrolysis has been confirmed even when isocitrate, oxalic acid, tartaric acid, malic acid and an aqueous lactic acid solution are used in addition to citric acid. Similarly, it has been confirmed that the hemicellulose in the rice husk is hydrolyzed by immersion in an aqueous citric acid solution to produce furfural.

  From the above results, it is recognized that the high molecular weight carbohydrates such as cellulose and hemicellulose contained in the biomass are caused to change the molecular structure by the carboxylic acid aqueous solution used in the present invention. As a result, high-molecular-weight carbohydrates are easily eluted in the solution, and the amount of carbon remaining in the silica obtained when residues such as rice straw and rice husks after acid treatment are fired in the air is reduced, resulting in higher silica purity. Can be planned.

  Even in the purification of silica extracted from the residue after washing rice straw and rice husk in carboxylic acid aqueous solution as described above, the use of carboxylic acid aqueous solution including citric acid and oxalic acid is The structure change is promoted, and as a result, the silica purity after firing is improved. The concentration of the carboxylic acid aqueous solution used at that time is preferably 1% to 20%.

(4) Purification of heavy metal contaminated soil Plants grow in contaminated soil containing heavy metal elements, absorb heavy metal elements in the plants, and further separate the plants from the soil, thereby reducing the amount of heavy metal elements contained in the soil. Can be reduced. The plant or biomass separated from the soil is subjected to the above-described immersion / washing treatment with a carboxylic acid aqueous solution, subsequent water washing treatment, and subsequent heat treatment. When biomass is heated in an air atmosphere to produce silica, the silica may be sprayed on the contaminated soil. Silica has good solubility, has a soluble silica component of 70% or more, and has performance equivalent to that of commercially available silica gel fertilizer.

  As biomass materials, rice shoots and leaves with different cadmium contents were prepared. This sample was obtained from the Akita Prefectural Agriculture, Forestry and Fisheries Technology Center.

  Each rice straw sample was immersed in a citric acid aqueous solution heated and maintained at 80 ° C. for 15 minutes, washed with tap water, and then dried at 100 ° C. in an air atmosphere to remove moisture. Weigh each 1g of the raw rice straw and the rice straw sample that has been treated with an aqueous citric acid solution, place it in 10ml of hydrofluoric acid and heat it to dissolve the Si component in the rice straw. Evaporated. Further, sulfuric acid was added to decompose the organic matter. Then, after heating and evaporating sulfuric acid, nitric acid was added, and it adjusted to 100 mL, and Cd content was measured by the ICP emission spectrometry. The results are shown in Table 1.

  As a result of washing treatment in an aqueous citric acid solution having an appropriate concentration specified in the present invention, the Cd content in any rice straw sample was below the lower limit of analysis (<1 ppm), and the Cd content from rice straw was reduced. It was confirmed that complete discharge and removal were possible. In particular, even when 90 ppm of Cd is contained, the Cd content in the rice straw after the acid cleaning treatment can be reduced to less than 1 ppm by setting the concentration of the citric acid aqueous solution to an appropriate range of 1% or more. A significant effect was observed. On the other hand, when washed in a 0.5% strength aqueous citric acid solution, the amount of Cd removed was not sufficient, and it was confirmed that Cd remained in the rice straw.

  About the shoot and leaf part (sample numbers 1 and 4) of rice having different cadmium contents used in Example 1, it was immersed in a 5% strength aqueous citric acid solution maintained at different temperatures for 15 minutes and washed with tap water. The sample was dried at 100 ° C. in an air atmosphere to remove moisture. Weigh each 1g of the raw rice straw and the rice straw sample that has been treated with an aqueous citric acid solution, place it in 10ml of hydrofluoric acid and heat it to dissolve the Si component in the rice straw. Evaporated. Further, sulfuric acid was added to decompose the organic matter. Then, after heating and evaporating sulfuric acid, nitric acid was added, and it adjusted to 100 mL, and Cd content was measured by the ICP emission spectrometry. The results are shown in Table 2.

  Cd can be removed by immersing rice straw containing Cd in an aqueous citric acid solution. However, as a result of washing treatment in an aqueous citric acid solution controlled at 40 ° C. or higher, in any rice straw sample, The Cd content was below the lower limit of analysis (<1 ppm), confirming a remarkable effect on Cd emission / removal from rice straw. It was confirmed that Cd remained slightly when washed in an aqueous citric acid solution having a water temperature of 23 ° C., which is close to room temperature.

  The stems and leaves (samples Nos. 1 and 4) of rice having different cadmium contents used in Example 1 were immersed in a 5% strength aqueous citric acid solution heated and maintained at 80 ° C., washed with tap water, and then air. The sample was dried at 100 ° C. in an atmosphere to remove moisture. At that time, the amount of Cd removal was evaluated by changing the immersion time. The Cd content analysis method was the same as in Example 1. The results are shown in Table 3.

  Under the above-mentioned acid treatment conditions, Cd contained in rice straw can be completely discharged out of the system and removed by performing acid treatment washing for 1 minute or more. On the other hand, it was confirmed that Cd remained slightly in the rice straw in a short washing process such as 15 seconds.

  The stems and leaves of rice (4 types of sample numbers 1 to 4) having different cadmium contents used in Example 1 were soaked in an aqueous oxalic acid solution (water temperature; 25 ° C., 50 ° C.) controlled at different concentrations for 15 minutes. After washing with tap water, the sample was dried at 100 ° C. in an air atmosphere to remove moisture. 1 g of each rice bran sample subjected to the oxalic acid aqueous solution washing treatment was weighed, placed in 10 mL of hydrofluoric acid and heated to dissolve the Si component in the rice straw, and then hydrofluoric acid was evaporated. Further, sulfuric acid was added to decompose the organic matter. Then, after heating and evaporating sulfuric acid, nitric acid was added, and it adjusted to 100 mL, and Cd content was measured by the ICP emission spectrometry. The results are shown in Table 4.

  When the liquid temperature of the oxalic acid aqueous solution is controlled at 25 ° C., the Cd content can be reduced by controlling the concentration to 1% or higher. However, if the liquid temperature is controlled to 50 ° C., the concentration is 1% or higher. Then, in all rice straw samples, the Cd content was below the lower limit of analysis (<1 ppm), and it was confirmed that Cd was completely discharged and removed from rice straw.

In Example 1, 30 g of rice straw treated with an aqueous citric acid solution at a concentration of 5% and 80 ° C. was baked in the air at 800 ° C. for 30 minutes, and the result of analyzing the SiO ₂ component contained in the obtained rice straw ash was analyzed. Table 5 shows. Samples that were not subjected to a washing treatment with an aqueous citric acid solution were also calcined and analyzed. As can be seen from the table, the SiO ₂ purity in all the samples was improved by the washing treatment with the citric acid aqueous solution, and showed a high value of 98% or more. On the other hand, the untreated sample was confirmed to be as low as about 92%.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to the thing of embodiment shown in figure. Various modifications and variations can be made to the illustrated embodiment within the same range or equivalent range as the present invention.

  INDUSTRIAL APPLICABILITY The present invention can be advantageously used as a method that can remove heavy metal elements contained in biomass with low cost and high efficiency and a purification method for heavy metal-contaminated soil.

It is a figure which shows the differential calorimetric analysis result when a rice husk is heated up from normal temperature to 1,000 degreeC. It is a figure which shows the pyrolysis gas chromatography mass spectrometry result regarding three types of samples heat-processed at 400 degreeC.

Claims (6)

Growing a plant on contaminated soil containing a heavy metal element, and absorbing the heavy metal element in the plant;
Separating the plant that has absorbed the heavy metal element from the soil;
A part or the whole of the plant separated from the soil as biomass, a step of immersing in a carboxylic acid aqueous solution and eluting the heavy metal element in the aqueous solution;
Removing the biomass after removal of the heavy metal element from the aqueous carboxylic acid solution;
Washing the biomass taken out from the carboxylic acid aqueous solution with water;
Heating the biomass after washing in air to produce silica;
And a step of spraying the silica onto the contaminated soil. The method for purifying heavy metal-contaminated soil according to claim 1 , wherein the plant growing on the heavy metal-contaminated soil is rice or wheat. The method for purifying heavy metal-contaminated soil according to claim 1 or 2 , wherein the carboxylic acid has a hydroxyl group. The method for purifying heavy metal-contaminated soil according to any one of claims 1 to 3, wherein the carboxylic acid is an acid selected from the group consisting of citric acid, isocitric acid, oxalic acid, tartaric acid, malic acid, and lactic acid. The temperature of the aqueous carboxylic acid solution is 50 ° C. or higher and 90 ° C. or lower,
The concentration of the carboxylic acid aqueous solution is 1% or more and 20% or less,
The method for purifying heavy metal-contaminated soil according to any one of claims 1 to 4 , wherein a retention time of the biomass in the carboxylic acid aqueous solution is 1 minute or more. The heating temperature in the air atmosphere is 300 ° C. or higher and 1100 ° C. or lower,
The method for purifying heavy metal-contaminated soil according to any one of claims 1 to 5 , wherein the silica is amorphous silica.

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