JP2014231058A - Method for removing heavy metal from plant biomass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an effective technique of using high-cadmium containing biomass that is harvested after soil purification by phytoremediation techniques.SOLUTION: After cadmium contained in plant biomass is dissolved into a solution by addition of sulfuric acid or hydrochloric acid to the plant biomass (rice straws, leaf vegetable and root vegetables) containing the high amount of heavy metal, cadmium in particular, dithiocarbamic acid compound and/or liquid chelate resin is added thereto to fix and precipitate the cadmium in an acid solution, and then the cadmium in the plant biomass is removed to produce a useful cadmium-free biomass resource.

Description

  The present invention relates to a method for efficiently removing heavy metals contained in plant biomass such as rice straw.

  It is well known that heavy metal components such as lead, cadmium, arsenic and zinc adversely affect the human body and the environment. Among them, cadmium is mixed into soil from abandoned mines and smelters and forms soil contaminated by paddy fields in the suburbs and downstream rivers. Rice grown in contaminated soil can accumulate cadmium at high concentrations.

  Under the Food Sanitation Law, the permissible cadmium concentration in brown rice has long been set at less than 1 ppm. However, in the trend that the review of the cadmium concentration in foods is being promoted by the World Health Organization (WHO), Japan revised the Food Sanitation Law standards in 2010 and implemented them in 2011 It is said that it should not contain more than 0.4 ppm as cadmium in brown rice and polished rice.

  As a method for reducing the cadmium concentration regulation in these agricultural products, for example, as a method for reducing the cadmium concentration in brown rice, there is a method (customer soil) in which uncontaminated soil is deposited in paddy fields. However, this land-based method is extremely expensive, so it is said that it is difficult to continuously put it into practical use from the economical aspect. In view of this, several methods have been proposed in the past as measures for removing heavy metals from paddy fields in contaminated soil containing heavy metal elements.

  There is a soil remediation technology (phytoremediation) using plants. Some plants, such as rice, soybeans, and rape, can absorb high concentrations of heavy metal elements, so these plants can be grown in contaminated soil areas where lead, cadmium, zinc, arsenic, etc. are present. Incorporate elements into plants.

  Cadmium in soil varies greatly in the amount absorbed by plants and crops depending on its chemical state. Therefore, for example, if the soil is reduced to an oxygen-deficient state, a method of completely irrigating the paddy field for 3 weeks before and after the heading period using a technique that makes it possible to keep the cadmium concentration in the brown rice low. It has been. In addition, it has been reported that when ferric chloride is added to paddy soil containing cadmium, cadmium adsorbed on the soil can be efficiently extracted by the action of the generated hydrogen ions and chloride ions (Patent Document 1).

  On the other hand, a method of eluting heavy metals in biomass by using carboxylic acids such as citric acid and tartaric acid has been proposed for biomass contaminated with heavy metals (Patent Document 2). In this method, high-purity amorphous silica is prepared from biomass from which heavy metals have been removed.

Japanese Patent Application Laid-Open No. 2005-163981 JP 2009-39601 A

  In the method of Patent Document 1, iron ions and chlorine ions are generated in a paddy field, and the chlorine ions react with a heavy metal such as cadmium to form a chloride. This method has a problem that the cost for purification becomes very high and excessive chlorine ions are absorbed by rice straw. Moreover, the carboxylic acid used in Patent Document 2 is relatively expensive.

  In general phytoremediation technology, plants grown in contaminated soil are harvested and then incinerated. Therefore, it is pointed out that incineration is likely to cause environmental problems such as the release of heavy metals such as cadmium into the atmosphere.

  It would be desirable to have a method for removing heavy metal elements at low cost and efficiently from plants that accumulate heavy metal elements at high concentrations. Moreover, the product (plant biomass from which heavy metal elements have been removed) resulting from the method may be in a form suitable as a raw material for producing energy and useful substances.

  On the other hand, it is known that the internal organs of some seafood such as scallops and squid midgut lines contain cadmium at a relatively high concentration. For this reason, in order to dispose of the product, it is usually necessary to incinerate it as industrial waste. However, these internal organs have a high protein content and can be a very useful raw material for cultured fish or livestock feed and fertilizer. Therefore, if there is an effective means for removing cadmium from these internal organs, effective utilization of the internal organs of fish and shellfish as feed and fertilizer can be expected.

The present invention provides the following.
[1] A process of treating a biomass (plant biomass or animal biomass) containing 0.4 mg / kg or more of a heavy metal with an aqueous solvent containing sulfuric acid and / or hydrochloric acid, and eluting the heavy metal into the solvent;
Separating the aqueous solvent-treated product into a solid residue and a liquid heavy metal-containing liquid;
A method for removing heavy metals in biomass, comprising adding a liquid chelating agent to the obtained heavy metal-containing liquid and collecting the heavy metals.
[2] The liquid chelating agent is added after adjusting the pH of the obtained heavy metal-containing liquid to exceed 4, and the liquid chelating agent contains a water-soluble compound having a dithiocarbamic acid group the method of.
[3] The method according to [1] or [2], wherein the heavy metal is any one selected from the group consisting of cadmium, mercury, copper, lead and zinc.
[4] The method according to any one of [1] to [3], wherein the aqueous solvent contains 1.5% or more of sulfuric acid or 4.0% or more of hydrochloric acid.
[5] The method according to any one of [1] to [4], comprising a step of washing the residue with a washing solution.
[6] The method according to [5], wherein the cleaning liquid is water or a dilute acid aqueous solution.
[7] The method according to [5] or [6], wherein 2.5 to 50 parts by weight of an aqueous solvent and 2 to 40 parts by weight of a cleaning solution are used with respect to 1 part by weight of biomass (for example, plant biomass).
[8] The method according to any one of [1] to [7], wherein the biomass is derived from a gramineous plant, a legume plant, a cruciferous plant, or a asteraceae plant.
[9] A method for removing contamination from soil, comprising the step defined in any one of [1] to [8], wherein plant biomass is cultivated in soil contaminated with heavy metals.
[10] A method for producing a raw material for production of energy or useful substances, comprising the step defined in any one of [1] to [8].

  According to the present invention, heavy metals can be effectively eluted and transferred to the solution side from plant biomass containing heavy metals (particularly cadmium) using sulfuric acid or hydrochloric acid. Furthermore, cadmium can be chelated and removed effectively and efficiently from the obtained heavy metal solution.

  The heavy metal-removed residue obtained by the present invention and the heavy metal-chelated solution can be used as a heavy metal-free raw material for the production of energy or useful substances.

  According to the present invention, it is possible to perform effective treatment of contaminated soil using phytoremediation, specifically treatment of contaminated soil in which plants produced by phytoremediation are also available as biomass resources.

  The present invention can also be implemented for biomass other than plant biomass. When carried out on animal biomass, it is possible not only to remove heavy metals but also to prepare solutions rich in amino acids.

Extraction of cadmium from rice straw (long perfumed grains) with H ₂ SO ₄ . The cadmium concentration in the supernatant after heat treatment of rice straw with an acid solution is shown. Cadmium extraction from rice straw (long perfumed grains) with HCl. The cadmium concentration in the supernatant after heat treatment of rice straw with an acid solution is shown. Removal of cadmium from plant biomass by heat treatment with sulfuric acid solution and washing treatment with water or dilute sulfuric acid solution. The cadmium concentration of the residue after the sulfuric acid treatment and the residue after the washing was shown. Dynamics of cadmium by sulfuric acid treatment and washing treatment in rice straw with different cadmium contents. Transfer rate of cadmium to liquid phase when rice straw with different cadmium contents was treated with sulfuric acid and washed. Removal of cadmium ions from acid extract of plant biomass by addition of dithiocarbamate compound. Removal of cadmium ions from acid extract of plant biomass by adding liquid chelating agent. Removal of cadmium ions from acid extract of rice straw containing cadmium. Colored state of acid extract of rice straw containing cadmium. Cadmium concentration in enzymatic saccharification residue when bioethanol is produced from rice straw containing cadmium. The cadmium concentration in the enzymatic saccharified solution when bioethanol is produced from rice straw containing cadmium. Amount of cadmium extracted by shaking for 30 minutes after addition of dilute sulfuric acid solution. The amount of cadmium extracted into the solution by heat treatment at 120 ° C for 1 hour. The amount of cadmium in the extract adjusted to pH after heat extraction. The amount of cadmium in the extract after cadmium removal treatment. Total analysis by nitric acid decomposition. Amount of Cd extracted when processed at a solid-liquid ratio of 1: 2. Difference in extraction rate of cadmium by shaking treatment or heat treatment by adding 1: 2 dilute sulfuric acid Cadmium concentration in the solution after pH adjustment and cadmium removal treatment after dilute sulfuric acid addition and heat treatment. The concentration of each amino acid in the extract (ppm) during the extraction of dilute sulfuric acid with a solid-liquid ratio of 1: 2. Amino acid content (g) obtained from 1 kg (wet weight) of raw materials

  In the present invention, when the numerical range is represented by “n to m”, the range includes the values n and m at both ends unless otherwise specified. Regarding the present invention, the values indicated by “x%” and the ratio “y: z” are values calculated based on the weight, unless otherwise specified.

  In the present invention, the term “biomass” includes plant biomass, animal biomass, and microbial biomass. In the present invention, when referring to the concentration of cadmium contained in plant biomass, the concentration is a value calculated based on the dry weight of plant biomass, unless otherwise specified. Moreover, regarding plant biomass in the present invention, when expressing weight, the weight is based on dry matter (dry weight), unless otherwise specified. In the present invention, when referring to the concentration of cadmium contained in animal biomass, unless otherwise specified, the concentration is a value calculated based on the weight of the animal biomass containing moisture (wet weight or wet weight). It is. Moreover, regarding animal biomass in the present invention, when the weight is expressed, the weight is a weight including moisture (wet weight) unless otherwise specified.

  In the present invention, in order to remove heavy metals from plants containing heavy metals, a relatively small amount of sulfuric acid or hydrochloric acid is used to remove heavy metals mainly from cadmium from biomass represented by solid plant biomass. The present invention relates to a technique for completely transferring to the solution side.

The method of the present invention comprises the following steps:
(1) A step of treating biomass containing 0.4 mg / kg or more of heavy metal with an aqueous solvent containing sulfuric acid and / or hydrochloric acid to elute the heavy metal in the solvent;
(2) A step of separating the aqueous solvent-treated product into a solid residue and a liquid heavy metal-containing liquid;
(3) A step of collecting a heavy metal by adding a liquid chelating agent to the obtained heavy metal-containing liquid.

  Step (1) is a step of eluting heavy metals from biomass into a solvent. The aqueous solvent used includes sulfuric acid and / or hydrochloric acid. Examples of preferred solvents are sulfuric acid aqueous solution or hydrochloric acid aqueous solution. When sulfuric acid is used, the concentration can be 0.50% or more, preferably 1.00% or more, and more preferably 1.25% or more. From the viewpoint of transferring heavy metal to 95% or more in an aqueous solvent, it is preferably 1.50% or more, more preferably 1.75%, and further preferably 2.00%. When hydrochloric acid is used, the concentration can be 1.00% or more, preferably 2.00% or more, and more preferably 2.50% or more. From the viewpoint that 95% or more of the heavy metal is transferred into the aqueous solvent, it is preferably 4.00% or more, more preferably 4.39%, and even more preferably 6.58%. The acid concentration can be set to a concentration suitable for decomposing polysaccharides contained in biomass from the viewpoint that a product with a reduced heavy metal content is used for energy or useful substance production.

  A person skilled in the art can appropriately design the ratio between the plant biomass and the aqueous solvent in the step (1) in consideration of the extraction efficiency, the amount of the heavy metal-containing liquid generated, and the like. For example, when sulfuric acid having a concentration of 2.00% or more and / or hydrochloric acid having a concentration of 6.50% or more is used, 2 to 20 parts by weight can be used with respect to 1 part by weight of plant biomass, and 2.5 to 15 parts by weight can be used. It is preferable to use 3 to 10 parts by weight.

  A person skilled in the art can appropriately design the ratio of the animal biomass and the aqueous solvent in the step (1) in consideration of the extraction efficiency, the amount of the resulting heavy metal-containing liquid, and the like. For example, when sulfuric acid having a concentration of 1.00% or more is used, 1.0 to 20 parts by weight can be used, preferably 1.5 to 15 parts by weight, and preferably 1.8 to 10 parts by weight with respect to 1 part by weight of animal biomass. It is more preferable.

  Step (1) can be carried out at ambient temperature, but can be carried out under heating, for example, at 80 to 121 ° C. You may pressurize as needed. The treatment time depends on the temperature and can be appropriately designed in consideration of the extraction rate. However, when the treatment time is 80 to 121 ° C., it can be several minutes to several hours, for example, 10 to 240 minutes.

  Step (2) is a step of separating the aqueous solvent-treated product into a solid residue and a liquid heavy metal-containing liquid. As a means for solid-liquid separation, a conventional method used for the same purpose can be appropriately applied. Examples of conventional solid-liquid separation means include sedimentation separation, filtration separation (membrane separation), and centrifugal separation.

  The obtained liquid phase, that is, the heavy metal-containing liquid is subjected to step (3).

  The resulting solid phase, i.e. the residue, may be subjected to a washing step. When carrying out the cleaning step, it is preferable to use water or dilute acid aqueous solution as the cleaning liquid. One preferred case of performing the washing step is when the target is plant biomass.

  When carrying out the washing step, the ratio between the residue and the aqueous solvent can be appropriately designed by those skilled in the art in consideration of the extraction efficiency, the amount of the heavy metal-containing liquid produced, and the like. For example, when sulfuric acid having a concentration of 2.00% or more and / or hydrochloric acid having a concentration of 6.50% or more is used, 2 to 20 parts by weight can be used with respect to 1 part by weight of plant biomass, and 2.5 to 15 parts by weight can be used. It is preferable to use 3 to 10 parts by weight. In order to reduce the heavy metal content to 0.2 ppm or less per dry weight of the finally obtained residue, when step (1) is carried out using 1% by weight of the starting biomass as a solvent with a sulfuric acid solution having a concentration of 1.5% or less Is preferably washed with 2 parts by weight (for example, 4 parts by weight) of 1% or less (preferably 0.5% or more) dilute sulfuric acid solution, and when step (1) is carried out with 1.6% or more sulfuric acid solution It is preferable to wash with 1.5 parts by weight (for example, 3 parts by weight) of water or dilute sulfuric acid (for example, 1% or less, preferably 0.5% or less, for example, 0.25% or less). It is preferable that the liquid phase obtained in step (1) and the washing liquid obtained from the washing step are combined and designed to be 20 parts by weight or less with respect to 1 part by weight of the starting plant biomass, and 15 parts by weight or less. More preferred is 10 parts by weight or less.

One of the optimal conditions is to add 1.5-3% sulfuric acid at 1: 4-8 (w / w) to plant biomass, and after processing at 100-130 ° C for 30-120 minutes, The residue obtained by separation is washed with 2 to 6 times the amount of pure water of the starting plant biomass. Under these conditions, cadmium can be sufficiently removed with a solution of 10 times the amount (weight) of plant biomass containing cadmium.
The cleaning liquid can be subjected to the next step (3) in addition to the heavy metal-containing liquid.

  Step (3) is a step of collecting a heavy metal by adding a liquid chelating agent to the obtained heavy metal-containing liquid (which may contain a cleaning liquid). When removing heavy metals from ordinary wastewater, it is effective to use a chelate resin in which a chelate group is bound to a solid resin. This is because the chelate resin can selectively adsorb heavy metals and can be easily removed from the liquid phase. However, according to the study by the present inventors, in an acid extract of biomass that is considered to contain a large amount of organic matter-based lysate, it is more efficient to use a heavy chelating agent instead of a chelating resin. It turns out that it can collect.

  Therefore, a liquid chelating agent is used in the method of the present invention. In the present invention, “liquid chelating agent” refers to a chelating agent in which an active ingredient having a chelate-forming group is water-soluble, unless otherwise specified. The liquid chelating agent is typically in the form of a solution in which a water-soluble active ingredient is dissolved in an aqueous solvent, but may be a water-soluble substance itself (solid) as an active ingredient.

In the method of the present invention, among liquid chelating agents, in particular, metal salts of dithiocarbamic acid, water-soluble polymer compounds having dithiocarbamic acid groups, and metal salts of compounds having two or more dithiocarbamic acid groups in the molecule (specifically Include potassium salt of diethyldithiocarbamate, metal salt of N ₁ , N ₂ , N ₃ , N ₅ -tetra (dithiocarboxy) tetraethylenepentamine), metal salt of piperazine bisdithiocarbamate (eg dipotassium = piperazine-1 , 4-dicarbodithioate), diethylamine chelating agent (for example, potassium = diethylamine-N-carbodithioate), sodium sulfide, sodium hydrosulfide, thiol compound (for example, water-soluble polymer compound having thiol group ( For example, an average molecular weight of 80,000 to 120,000)) can be suitably used. In the context of the present invention, when referring to a metal salt, an alkali metal salt, an alkaline earth metal salt, or a salt with ammonium can be used. Of these, lithium, sodium, potassium, magnesium, calcium, barium, ammonium, and the like. The salt of is preferably used.

  According to the study by the present inventors, it has been confirmed that it is particularly preferable to use a liquid chelating agent having a thiocarbamic acid group as a chelate-forming group. Examples thereof include metal salts of dithiocarbamic acid, water-soluble polymer compounds having a dithiocarbamic acid group (for example, an average molecular weight of 80,000 to 120,000), and thiocarbamate. More specific examples include Taichelate F-1 (Daimei Chemical Co., Ltd.) and a metal salt of dithiocarbamic acid.

A metal salt of dithiocarbamic acid (sometimes referred to as dialkyldithiocarbamic acid) is represented by the following structure.

R ₁ and R ² are lower alkyl groups or aromatic groups, and may be the same group or different groups. As specific examples, a sodium salt of dimethyldithiocarbamic acid, a sodium salt of diethyldithiocarbamic acid, and a sodium salt of dibenzyldithiocarbamic acid can be given.

When using a liquid chelating agent having a thiocarbamic acid group as a chelating group, it is preferable that the pH of the solution is adjusted before the addition. The preferred pH after adjustment is 4-10. For pH adjustment, NaOH, Mg (OH) ₂ , Ca (OH) ₂ , FeCl ₂ , FeCl ₃ , Al ₂ (SO ₄ ) ₃ or the like can be used. A person skilled in the art can appropriately determine the amount of the liquid chelating agent added. A more appropriate addition amount can be determined by preparing and confirming the simulated liquid in advance. For example, when the metal salt of the above-mentioned dithiocarbamic acid (also referred to as dialkyldithiocarbamic acid) is used, it can be added so that the concentration in the solution is 0.01 to 10.0 mM, preferably 0.1 to 1.0 mM.

  Step (3) can be performed at ambient temperature. You may implement, stirring as needed. The treatment time can be appropriately designed in consideration of the heavy metal concentration, the chelating agent concentration, and the chelate rate, but can be several minutes to several hours, for example, 5 to 240 minutes with stirring.

  By the step (3), the heavy metal contained in the heavy metal-containing liquid obtained from the step (2) is collected by chelate. If necessary, a polymer flocculant or the like may be added to the chelate-collected solution. The heavy metal collected by chelation can be removed by solid-liquid separation means of the prior art, for example, sedimentation separation, filtration separation (membrane separation), centrifugation, and the like.

  In the method of this invention, the plant body used for the phytoremediation can be used as plant biomass. Plants include gramineous plants (eg, rice, sorghum, corn, wheat), legumes (eg, soybeans), asteraceae plants (eg, sunflower), cruciferous plants (eg, octopus, rapeseed, rapeseed) , Brassica juncea, Brassica carinata), Scots family (e.g. Moedidae), Amaranthaceae (e.g. Amaranthus), or Rubiaceae (e.g. Akaza). Preferably, the plant biomass used in the present invention is derived from a grass family plant, a legume plant, a cruciferous plant, or a asteraceae plant. Other examples include Indian Mustard, Gunby Nazuna, Alfalfa, Aseitaka Alaska, Kenaf, Poplar, Tall Fescue, Water Hyacinth, Mung Bean, Reed, Pine, Spruce and the like. Also, genetically modified plants can be used. There is no restriction | limiting in particular in the cultivation method of a plant.

  In the method of the present invention, seafood can be used as animal biomass. The seafood may be fish (cartilaginous fish, teleosts), jawless (lampfish), shellfish, cephalopods, echinoderms, crustaceans, cnidarians, and the like.

  The plant biomass used in the method of the present invention can be derived from any part of the plant body. The whole plant body may be used, only the above-ground part may be used, or a part of the plant body such as a stem, a leaf, a flower, a fruit, a seed), or a rice husk may be used. There are no particular restrictions on the collection and harvesting methods. The collected and harvested plant biomass may be dried.

  The animal biomass used in the method of the present invention can be derived from any part of the animal body. The whole animal body may be used, and a part of the animal body, for example, the internal organs may be used. Preferable examples of animal biomass used in the present invention are internal organs such as scallop, squid and octopus (for example, midgut line, gill, gonad).

  The present invention is particularly excellent for removing cadmium. In particular, phytoremediation is excellent in removing cadmium from the viscera of fish and shellfish containing a large amount of cadmium and plant biomass such as rice straw and leafy vegetables that have absorbed cadmium in the soil. In the present invention, a minimum amount of sulfuric acid or hydrochloric acid is used to dissolve and release cadmium in biomass effectively and efficiently, and then dithiocarbamate compound and / or liquid chelating agent (for example, tie chelate) is used. By adding to the solution, the cadmium ions in the solution are reacted, adsorbed and precipitated with these compounds, so that the cadmium ions in the solution can be highly removed.

  According to the present invention, cadmium can be extracted from almost 100% animal biomass into the liquid phase by sulfuric acid addition and heat treatment. Almost cadmium-free animal biomass can be obtained by solid-liquid separation or dehydration using a filter press or the like. It is known that the internal organs of seafood contain extremely abundant proteins, and can be used as feed or fertilizer. Moreover, the solution derived from animal biomass from which the cadmium ions in the solution are highly removed can contain various amino acids derived from animal biomass. Therefore, the effective use as liquid fertilizer can be expected. Such a solution may also be used as a raw material for amino acids.

  Examples of the present invention will be described below. The scope of the present invention is not limited to the following examples.

  As a plant biomass sample containing cadmium, a pulverized product of rice straw (variety: long-scented cereal, cadmium concentration: 33.4 mg / kg DW) was used. Add an acid solution prepared by diluting sulfuric acid or hydrochloric acid to the sample so that it will be 1: 5 (w / w) (specifically, add 10 g of acid solution to 2 g of crushed rice straw) The mixture was thoroughly stirred after heat treatment at 120 ° C. for 60 minutes. The concentration of free cadmium in the supernatant obtained by centrifugation at 10,000 × g for 10 minutes was measured with an ICP-MS apparatus (X series II, Thermo Fisher Scientific Co., Ltd.).

  Fig. 1 shows the analysis results using sulfuric acid, and Fig. 2 shows the analysis results using hydrochloric acid. Sulfuric acid at a concentration of 1.5% (0.54N) or higher and hydrochloric acid at a concentration of 4.39% (0.5N) or higher was able to liberate 95% or more of cadmium from plant samples. Therefore, in order to remove 95% or more of cadmium from plant biomass containing cadmium, which is the subject of this study, the acid concentration is preferably 1.5% or more for sulfuric acid and 4.39% or more for hydrochloric acid. It became clear.

  As plant biomass samples containing cadmium, rice straw (variety: long cereals, cadmium content 33.37mg / kg DW) was pulverized and the conditions for completely releasing cadmium into the extract were investigated. .

30 ml of a 0.5%, 1%, 1.5%, or 2% sulfuric acid solution was added to 5 g of the above ground rice straw, and autoclaved at 120 ° C. for 60 minutes. After the autoclave treatment, a 20 ml cylindrical funnel was filled with the autoclave treated product while sucking and filtering the filtrate by using a glass filter for a φ25 mm filter manufactured by Shibata Kagaku Co., Ltd. under reduced pressure by a dry aspirator. As the filter paper, GA-55 (glass fiber filter paper, pore diameter 0.6 μm) manufactured by Advantech Co., Ltd. was used. During filling, the residue was compressed so that the volume of the residue on the filter paper was 10 cm ³ . When the filtrate stopped leaching, the residue was washed with pure water, 0.5% sulfuric acid or 1.0% sulfuric acid (5 ml × number of washings). The washed residue was dried, decomposed with nitric acid, and the cadmium content was quantified by ICP-MS.

  The results are shown in Figure 3. In order to reduce the cadmium content to 0.2 ppm or less per dry weight, when heat-treated with a 1.5% sulfuric acid solution, it is preferable to wash with 20 ml or more of a 0.5% or 1% dilute sulfuric acid solution, and a 2% sulfuric acid solution. In the case of treatment, it was found that it is preferable to wash with 15 ml or more of dilute sulfuric acid solution. One of the optimal conditions is that the plant biomass containing cadmium is added with 2% sulfuric acid at 1: 6 (w / w), autoclaved at 120 ° C for 60 minutes, and then separated into solid and liquid. The residue obtained is washed with 4 volumes of pure water (specifically, 20 g of 4 times the amount of 5 g of raw biomass). Under these conditions, cadmium can be sufficiently removed with a solution of 10 times the amount (weight) of plant biomass containing cadmium.

  Using the optimum conditions shown in Example 2, crushed rice straw (long-scented cereal, cadmium concentration in each lot J90A; 46.2 mg / kg, JM-01; 33.4 mg / kg, HR-01; 23 mg / kg, Cadmium was extracted from HN-02; 12.6 mg / kg, and normal cultivated Akitakomachi cadmium concentration 0.41 mg / kg). The residue after the removal operation was decomposed with nitric acid, and the amount of cadmium remaining in the residue and the amount of cadmium transferred to the solution were measured by ICP-MS.

  The results are shown in FIG. The removal rate of cadmium at that time is shown in FIG. It was shown that cadmium can be removed with high efficiency regardless of the variety.

  As the next step of extracting cadmium from plant biomass, the removal of cadmium from an acid solution containing cadmium was examined.

  Dithiocarbamate groups are known to form strong complexes with cadmium and zinc ions. Usually, a chelate resin in which this functional group is introduced into a bead-shaped resin is used to remove these heavy metals. However, in a system for treating plant biomass containing cadmium, it is discarded under the conditions we have experimented with. Cadmium could not be removed to a level that would not cause a problem in material treatment (see comparative example below).

100 ml of 2% sulfuric acid was added to 15 g of rice straw containing cadmium and autoclaved at 120 ° C. for 60 minutes. Distilled water (100 ml) was added to the treatment liquid, mixed, and filtered through glass fiber filter paper (GA-55) to prepare an acid extract (1% H ₂ SO ₄ solution). The acid extract was adjusted to pH 4.0-7.0. We investigated whether solid-liquid separation is possible by adding a low-molecular dithiocarbamate compound to the acid extract at each pH, forming a complex with cadmium ions, and then precipitating.

  Dimethyldithiocarbamate (Sodium NN-Dimethyldithiocarbamate Dihydrate, Wako Pure Chemical Industries, Ltd.), Diethyldithiocarbamate Trihydrate (Wako Pure Chemical Industries, Ltd.), Dibenzyldithiocarbamate Hydrate (Tokyo Chemical Industry) Was added so that the concentration in the solution would be 0.1 mM, 0.25 mM, 0.5 mM, and 1.0 mM, and stirred. Then, cadmium ions contained in the supernatant were measured after centrifugation at 10000 × g for 10 minutes.

  The results are shown in FIG. The active functional group is the same, but it is thought that when the complex was formed with the cadmium ion, the result was different depending on whether the precipitate was formed or the dissolved state was maintained. At pH 5.5, the addition of dibenzyldithiocarbamate reduced the cadmium ion concentration to 10 ppb or less. In particular, at a concentration of 0.5 mM or higher, it was possible to reduce the cadmium ion concentration to 3 ppb or lower, which is the environmental standard for public waters and groundwater.

  Similar to the dithiocarbamate compound, a liquid chelating agent was added to a plant sample extract containing cadmium, and a method of removing cadmium from the solution was attempted.

100 ml of 2% sulfuric acid was added to 15 g of rice straw containing cadmium and autoclaved at 120 ° C. for 60 minutes. Distilled water (100 ml) was added to the treatment liquid, mixed, and filtered through glass fiber filter paper (GA-55) to prepare a cadmium-containing solution (1% H ₂ SO ₄ ). Add and mix liquid chelating agent (Taichel F-1, Daimei Chemical Co., Ltd.) to 100ml of solution adjusted to each pH, perform solid-liquid separation by centrifugation at 10,000 xg for 5 minutes, and adjust cadmium concentration in the supernatant. Was measured. The amount of liquid chelate added was 0.05%, 0.03%, 0.02%, 0.015%, and 0.01% with respect to the amount of liquid added.

  The analysis results are shown in FIG. When the pH of the solution was adjusted to 5.0 or 5.5 and a liquid chelating agent was added, it was possible to stably precipitate cadmium ions and reduce the concentration in the solution to 10 ppb or less. The concentration of the liquid chelating agent added was considered to be sufficient at 0.01%.

  Moreover, when other dissolved cations were examined, most of Zn was removed at pH 5.5. However, other ions were hardly affected, and it was considered that Cd and Zn (Cu) could be removed specifically. At that time, the weight of the precipitate formed by the addition of the liquid chelating agent was extremely small.

Comparative example

  The removal of cadmium ions from the acid extract of rice straw containing cadmium was investigated using a chelating resin.

100 ml of 2% sulfuric acid was added to 15 g of rice straw containing cadmium and autoclaved at 120 ° C. for 60 minutes. Distilled water (100 ml) was added to the treatment liquid, mixed, and filtered through glass fiber filter paper (GA-55) to prepare an acid extract (1% H ₂ SO ₄ solution). To 100 ml of the resulting solution, as a bead-like chelating resin, Diaion (registered trademark) Sepabeads (registered trademark) CR11 and CR20 (Mitsubishi Chemical Corporation), Sumitchel MC600, MC700, MC760, and MC960 (Sumitomo Chemtex Co., Ltd.) Company) and Q10R (Daiichi Kasei Co., Ltd.), each with 1% of the solution weight as dry weight, adjusted pH with NaOH, and the concentration of heavy metal ions dissolved in the solution at each pH is ICP- Quantified by MS. Moreover, after adjusting to each pH, the cadmium concentration contained in the supernatant when each resin was added was also measured.

  The results are shown in FIG. When any resin was added, most of the cadmium in the solution could be removed, but could not fall below 10 ppb.

  In the bead-like resin, metal ions are often adsorbed only on or near the surface of the resin because of its structure, and it is considered that the metal ions hardly penetrate into the inside. Although the resin is processed into a porous shape, as far as the data of this comparative example is seen, the reason why the cadmium could not be sufficiently absorbed and removed is that the cadmium ion formed a complex with an organic substance. It is conceivable that the molecular weight is larger than that of ions alone, so that the resin cannot enter the resin and repels the resin due to the charge.

  After acid extraction treatment, the weight of rice straw is about 55% before treatment, and nearly 45% dissolves in the acid extract. Further, as shown in FIG. 10, the acid extract is colored brown, and it is considered that an organic substance having a chelating action is considerably dissolved. From the experimental results using bead-shaped chelate resins, the acid extract of plant biomass contains various organic substances that form complexes with cadmium ions together with metal ions that compete with cadmium ions. Expected to be bound to ions.

  A substance having a chelating power stronger than that of the organic substance is desirable for separating and adsorbing the cadmium from the dissolved organic substance by binding to the cadmium ion. It is also desirable that such a chelating substance also allows cadmium to be desorbed from the organic substance and adsorbed by chelating action so that it can quickly precipitate in the solution and be removed from the solution by solid-liquid separation.

While the bead-like chelate resin is a technology that separates cadmium by transferring it to a solid phase called chelate resin by solid-liquid distribution, the liquid chelate substance adsorbs cadmium ions from the liquid-liquid distribution state. It is more preferable for treating an acid extract from plant biomass containing cadmium because it forms a precipitate immediately after forming the complex, that is, becomes a solid and can be easily removed from the solution.
[Reference example]

Each rice straw used in Example 3 (variety: long-scented cereal, cadmium concentration in each lot J90A; 46.2 mg / kg, JM-01; 33.4 mg / kg, HR-01; 23 mg / kg, HN-02; 12.6 100 ml of 2% H ₂ SO ₄ was added to 15 g of 15 mg (kg / kg, cultivated Akitakomachi cadmium concentration 0.41 mg / kg), and heat treatment was performed at 120 ° C. for 60 min. Without performing solid-liquid separation by filtration, 100 ml of pure water was added, and the pH was adjusted to 6.5 with sodium hydroxide. Cellulase (Mecellase, Meiji Pharma Co., Ltd.) and hemicellulase (Hemicellulase “Amano” 90, Amano Enzyme Co., Ltd.) were added in an amount of 1 g each (10% enzyme solution was prepared and added), 50 ° C. The cellulose was saccharified while shaking at 150 rpm for 24 hours. After the enzyme treatment, the solution was subjected to suction filtration with glass fiber filter paper (GA-55), and solid-liquid separation was performed on the enzyme saccharification residue and the enzyme saccharified solution.

  The enzyme saccharified solution was added with pre-cultured yeast and subjected to ethanol fermentation. The obtained fermented liquor was collected by centrifugation and then separated into ethanol and distillation residue by distillation using a rotary evaporator.

  The concentration of metal ions including cadmium in the samples of the above steps was measured. The enzyme saccharification residue and yeast were dried and ground, then weighed accurately in a glass test tube, subjected to nitric acid decomposition, and the cadmium content was measured by ICP-MS. The enzyme saccharified solution and the distillation residue were diluted, and the concentration of cadmium ions was measured by ICP-MS.

  The results are shown in FIGS. 10 and 11. The cadmium concentration contained in the enzymatic saccharification residue was 2.4 to 3.4 times the concentration contained in the raw rice straw. In the bioethanol production process, hemicellulose and cellulose are saccharified and eluted from plant biomass, and the solid content after saccharification is 34 to 38% of the raw material, but cadmium is concentrated in enzymatic saccharification residue It became.

On the other hand, it was found that when the cadmium concentration of the raw rice straw is high, the cadmium concentration of the enzyme saccharified solution is also high. When rice straw with a cadmium concentration of 12.6 mg / kg was used as a raw material, the cadmium concentration of the enzyme saccharified solution was 93.5 ppb. It was found that if rice straw with a higher concentration than this is used as a raw material, there is a high possibility that it would exceed the effluent standard of 100 ppb from the factory.

In addition, the cadmium concentration of the long residue cereal JM-01 (cadmium concentration 33.4mg / kg) was 0.29.
The cadmium content of the yeast was 2.79 mg / kg DW. In all cases, the cadmium concentration was high, and the treatment as industrial waste became a problem.

  As animal biomass samples containing cadmium, scallop uro (midgut line), gills, female gonads, and mid-gut line of squid were used to conduct cadmium removal tests from the internal organs of these seafood. After washing with water, the sample is pulverized with a miller until it becomes slurry, and weighed into a 50 ml centrifuge tube made of PP, and 1% or 2% sulfuric acid is added to this in 1: 1 (w / w), 1: 2, 1: It added so that it might become 3. The following operation was performed after the addition, and an extract was prepared from each. Using a reciprocating shaker, shaking was performed at 120 rpm for 30 minutes. After the shaking treatment, solid-liquid separation was performed by centrifugation at 10000 × g for 10 minutes, and the extract was filtered through No. 5C filter paper to obtain an extract.

  Moreover, the heat processing by an autoclave for 60 minutes was performed at 120 degreeC. After the heat treatment, solid-liquid separation was performed by centrifugation at 10,000 × g for 10 minutes to obtain an extract.

  For the extract obtained by heat treatment, adjust the pH to about 5.0 to 5.2, add tie chelate to the extracted solution after adjusting the pH to 0.1%, and stir and precipitate by centrifugation. Was removed. The cadmium concentration in the liquid was measured for the obtained supernatant. The measurement was performed with an ICP-MS apparatus (X series II, Thermo Fisher Scientific Co., Ltd.) after timely dilution.

  The amount of Cd extracted by the shaking process is shown in FIG. Looking at the addition of dilute sulfuric acid with a solid-liquid ratio of 1: 2, it was revealed that 94% or more of cadmium was extracted from any sample by shaking alone (FIG. 18). This extraction rate is further increased by the heat treatment (FIG. 13). It was revealed that almost 100% of cadmium can be extracted and removed except for gonads with low cadmium concentration (Fig. 18). FIG. 14 shows the cadmium concentration after adjusting the pH of the extract obtained by the heat treatment. FIG. 15 shows the cadmium concentration in the extract after addition of liquid chelate (Tychelate) to 0.1% after pH adjustment and removal of the precipitate by stirring centrifugation. The extract contained almost no cadmium (FIG. 19), and it became clear that cadmium could be separated and removed almost completely as a precipitate.

  Moreover, the result (FIG. 17) about solid-liquid ratio 1: 2 is shown with the result of a total amount analysis (FIG. 16). The extraction effect was enhanced at 1: 2 or more, and cadmium removal treatment at about 1: 2 or more was considered preferable.

  The amount of free amino acid in the extract obtained by subjecting to a shaking treatment after addition of dilute sulfuric acid at a solid-liquid ratio of 1: 2, heat treatment, and heat treatment, pH adjustment and cadmium removal treatment with liquid chelate was quantified.

  As a result, it was shown that the extract contained amino acids at a high concentration and was extremely useful in the agricultural field such as liquid fertilizer (FIGS. 20 and 21).

  According to the present invention, cadmium can be efficiently dissolved and released from plant biomass such as rice straw and leaf vegetables that have highly absorbed cadmium in soil by using a small amount of sulfuric acid or hydrochloric acid, particularly by phytoremediation. be able to. Moreover, cadmium ions in the solution can be precipitated and removed by using an appropriate chelating agent thereafter. The present invention can be used in industries related to environmental remediation, soil treatment, and energy production from plant biomass. The method of the present invention is also effective for animal biomass, and the internal organs of fish and shellfish that have been treated as industrial waste can be effectively used as feed, fertilizer and the like according to the present invention.

Claims (10)

Treating biomass containing 0.4 mg / kg or more of heavy metal with an aqueous solvent containing sulfuric acid and / or hydrochloric acid, and eluting heavy metal in the solvent;
Separating the aqueous solvent-treated product into a solid residue and a liquid heavy metal-containing liquid;
A method for removing heavy metal in biomass, comprising adding a liquid chelating agent to the obtained heavy metal-containing liquid and collecting the heavy metal.   2. The method according to claim 1, wherein a liquid chelating agent is added after adjusting the pH of the obtained heavy metal-containing liquid to exceed 4, and the liquid chelating agent contains a water-soluble compound having a dithiocarbamic acid group.   The method according to claim 1 or 2, wherein the heavy metal is any one selected from the group consisting of cadmium, mercury, copper, lead, and zinc.   The method according to any one of claims 1 to 3, wherein the aqueous solvent contains 1.5% or more of sulfuric acid or 4.0% or more of hydrochloric acid.   The method according to any one of claims 1 to 4, comprising a step of washing the residue with a washing liquid.   6. The method according to claim 5, wherein the cleaning liquid is water or a dilute aqueous acid solution.   The method according to claim 5 or 6, wherein 2.5 to 50 parts by weight of an aqueous solvent and 2 to 40 parts by weight of a cleaning liquid are used with respect to 1 part by weight of biomass.   The method according to any one of claims 1 to 7, wherein the biomass is derived from a gramineous plant, a legume plant, a cruciferous plant, or a asteraceae plant.   A method for removing contamination from soil, comprising the step defined in any one of claims 1 to 8, wherein the plant biomass is cultivated in soil contaminated with heavy metals.   A method for producing a raw material for production of energy or useful substances, comprising the steps defined in any one of claims 1 to 10.