Method for recovering transition metal and production method for microorganism for use in same

Abstract

According to the present invention, a transition metal that is included in a material to be processed can be recovered by applying a positive potential to the material to be processed within a medium in the presence of a sulfur-oxidizing microorganism and dissolving at least a portion of the material to be processed in the medium. A culture obtained by culturing a sample such as rock collected from a seabed that has hydrothermal vents in a culture medium that includes thiosulfate can be used as the microorganism.

Description

Method for recovering transition metals and method for producing microorganisms used in this method.

[0001] The present invention relates to a method for recovering transition metals from a material to be treated, such as an alloy, and a method for producing microorganisms used in this method.

[0002] Many valuable metals are produced in limited countries, and the need for metal recycling is increasing from the perspective of obtaining materials stably without being affected by global conditions. As a method of metal recycling, many methods for recovering metals from various batteries have been studied. For example, Patent Document 1 discloses that a voltage was applied to electrodes containing nickel hydroxide and cobalt hydroxide in used nickel-metal hydride batteries to dissolve the cobalt and make it recoverable. On the other hand, metal materials such as stainless steel, which are widely used in various industrial materials and products, are often recycled by melting them in electric furnaces along with the raw materials. However, it has not been possible to extract valuable metals such as nickel contained in stainless steel from this recycling flow and utilize them in an upcycled manner.

[0003] Japanese Patent Publication No. 2025-40692

[0004] J. Japan Inst. Metals, Vo1.61, No. 6 (1997), pp, 486-493Materials Degradation (2022) 6:45; https: / / doi.org / 10.1038 / s41529-022-00254-0

[0005] The object of this invention is to provide a method for recovering transition metals contained in materials such as stainless steel and welded parts from materials to be processed.

[0006] Metals can be dissolved as ions in a liquid, allowing for element-selective recovery using various known methods. However, stainless steel, for example, has high corrosion resistance and rarely corrodes even in normal operating environments. Dissolving stainless steel by applying a potential, similar to the method described in Patent Document 1, requires the application of a high voltage, and it is not possible to recover specific metal elements. On the other hand, stainless steel is known to corrode in the presence of microorganisms, and dissolution has been confirmed in nature. However, while there have been incident cases of microbial-induced stainless steel corrosion at the field level, there have been very few successful examples of reproduction experiments at the laboratory level. In this context, Non-Patent Document 1 reports the selective dissolution of metal structures such as the γ phase. Furthermore, the present inventors have revealed the accumulation of electrochemically active microorganisms and microorganisms with sulfur-oxidizing ability through microbial community structure analysis in stainless steel corrosion in real environments (Non-Patent Document 2). The inventors of this invention focused on the fact that corrosion caused by microbial action can sometimes progress more rapidly than predicted by physicochemical factors. Based on the knowledge obtained from the above-mentioned literature, they investigated a method for dissolving metal products such as stainless steel and various alloys at an industrially usable rate using microorganisms and recovering specific metal elements, thereby completing the present invention.

[0007] Examples of typical embodiments of the present invention are shown below. <1> A method for recovering at least a portion of transition metals contained in a material to be treated, comprising applying a positive potential to the material in a medium in the presence of a microorganism having sulfur-oxidizing ability, thereby dissolving at least a portion of the material to be treated in the medium. <2> The method according to <1>, wherein the material to be treated comprises iron and nickel. <3> The method according to <2>, wherein the material to be treated is stainless steel and nickel is recovered. <4> The method according to any one of <1> to <3>, wherein the medium comprises thiosulfate. <5> The method according to <4>, wherein the microorganism is a culture obtained by culturing a marine sample in a medium containing thiosulfate. <6> The method according to any one of <1> to <5>, wherein the microorganism comprises Halomonas bacteria and nickel is recovered individually. <7> The method according to <6>, wherein the medium comprises thiosulfate and 1.2% to 15% by mass of NaCl. <8> The method according to any one of <1> to <5>, wherein the microorganism comprises at least one selected from the genus Thiobacillus and the genus Thiomicrospira, and nickel is dissolved in a medium as at least a part of the object to be treated. <9> A method for producing a microorganism for use in the method according to any one of <1> to <8>, comprising culturing a rock sample taken from the seabed where hydrothermal vents are present or a rock sample containing sulfide minerals in a medium containing thiosulfate. <10> The method according to <9>, comprising immersing the object to be treated in the culture obtained in the above culture and in a medium containing thiosulfate, and applying a positive potential to select a culture in which at least a part of the object to be treated is confirmed to be dissolved.

[0008] The present invention provides a method for recovering transition metals from a material to be treated and a method for producing microorganisms used in this method. According to one embodiment of the present invention, transition metals contained in a material to be treated can be recovered individually with high energy efficiency without requiring high temperature or high voltage.

[0009] The microbial community structure of rock samples and thiosulfate-containing inorganic salt cultures is shown. The microbial community structure of rock samples and thiosulfate-containing artificial seawater cultures is shown. The current value and electrical quantity changes in a potential holding experiment using thiosulfate-containing inorganic salt cultures are shown. The stainless steel test specimen electrodes after the completion of the potential holding experiment using thiosulfate-containing inorganic salt cultures are shown. The electrical quantity changes in a potential increase experiment in thiosulfate-containing artificial seawater cultures are shown. The stainless steel test specimen electrodes at the start of the experiment, during cultivation (no application), when +30 mV was applied, and after the application was completed in thiosulfate-containing artificial seawater cultures are shown. The LIBS analysis results for the test specimen (sound area near the weld) after the electrochemical test in thiosulfate-containing inorganic salt cultures are shown. The LIBS analysis results for the test specimen (corroded area) after the electrochemical test in thiosulfate-containing inorganic salt cultures are shown. The LIBS analysis results for the coating-sound area of ​​the test specimen after the electrochemical test in thiosulfate-containing artificial seawater cultures are shown. The LIBS analysis results for the corroded area of ​​the test specimen after the electrochemical test in thiosulfate-containing artificial seawater cultures are shown. The following shows the results of XRF analysis of the corroded portion of a specimen after electrochemical testing in thiosulfate-containing artificial seawater culture medium. The columns on the right side of the figure, with their numerical values, show the relationship between the intensity of fluorescent X-rays and the detected color. The color at the position of the middle arrow (green to yellow) indicates stronger X-rays than the color at the position of the bottom arrow (blue), and the color at the position of the top arrow (red) indicates the strongest X-rays among the three arrows. The following is an SEM image of the corroded portion of a specimen after electrochemical testing in thiosulfate-containing artificial seawater culture medium, observed from an oblique angle after cutting the sample perpendicular to the surface. The following is a stitched image of multiple SEM images of a sample observed from the cut surface after cutting the corroded portion of a specimen after electrochemical testing in thiosulfate-containing artificial seawater culture medium.

[0010] The present invention will now be described in detail. The following descriptions of constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments.

[0011] <<First Embodiment>> The first embodiment of the present invention is a method for recovering transition metals from a material to be treated, comprising applying a positive potential to the material in a medium in the presence of microorganisms to dissolve at least a portion of the material to be treated. In this case, a microorganism having sulfur-oxidizing ability is used.

[0012] The method according to the first embodiment makes it possible to utilize the phenomenon of microbial corrosion of metal materials such as stainless steel, which has conventionally been a cause of environmental pollution and economic loss, as a method for actively dissolving metal materials that require recycling. In other words, it becomes possible to rapidly dissolve metal materials and separate the metal elements that make up the metal materials, thereby diversifying the options for recycling metal materials. The method according to the first embodiment of the present invention includes a method for recovering transition metals contained in a material to be treated, one metal element at a time, and a method for selectively recovering a portion of the transition metals contained in a material to be treated, one metal element at a time. As specifically shown in the examples, in the method according to the first embodiment, selective dissolution of a portion of the metal elements that make up the material to be treated is possible by selecting microorganisms and media, and it is also possible to selectively recover the target metal elements by only performing a step that substantially implements the method according to the first embodiment as a step for separating metal elements.

[0013] <Object to be processed> The object to be processed in the method according to the first embodiment of the present invention contains at least a transition metal, and the transition metal contained in the object to be processed may consist of a single element or multiple elements. The multiple elements may be mixed at the atomic level, mixed at the metal crystal level, mixed at the metal microstructure level, or combined into a metal component. It is preferable that the object to be processed has a transition metal as its main component. It is preferable that the object to be processed contains the transition metal as a metal forming metallic bonds, and it is preferable that it has a metallic crystal structure. For example, it is preferable that the object to be processed does not contain the metal only as a metal salt. The object to be processed may also contain metals other than transition metals, or non-metallic resins, ceramics, etc., to the extent that conductivity is ensured.

[0014] As transition metals, first transition metals are particularly preferred, especially when present in an amount of 1.0% by mass or more relative to the total mass of the object to be treated. Titanium (Ti), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) are more preferred. When the object to be treated contains multiple metals, it is preferable that two or more of the multiple metals are first transition metals.

[0015] The object to be processed preferably has a shape that allows for the securing of wiring sections for applying a positive potential and conductive sections through which electricity flows. For example, it may be the original shape of the product that needs to be recycled, or it may be cut into a plate shape, rod shape, etc. It is also preferable to cut it in a way that increases the surface area in order to increase the wetted surface area that comes into contact with the medium.

[0016] Examples of materials to be processed include composite materials and alloys that combine materials consisting of different metal elements or different metal compositions. Examples of composite materials include structures in which a metal matrix is ​​joined with another metal material. Examples include dissimilar welding materials of titanium and stainless steel, and dissimilar welding materials of copper and stainless steel.

[0017] An alloy is a material obtained by melting and solidifying two or more metallic elements. It may also contain trace amounts (for example, 1.5% by mass or less) of nonmetals or metalloid elements in addition to metallic elements. Examples of metallic elements in an alloy include iron (Fe), nickel (Ni), cobalt (Co), chromium (Cr), and molybdenum (Mo). Examples of nonmetals and metalloid elements include carbon (C), oxygen (O), nitrogen (N), boron (B), phosphorus (P), sulfur (S), and silicon (Si). In one embodiment of the present invention, the alloy to be processed is preferably an iron alloy mainly composed of iron or a nickel alloy mainly composed of nickel, more preferably one containing iron and nickel, and even more preferably one containing iron, nickel, and chromium. Stainless steel is preferred as the alloy.

[0018] Stainless steel is an alloy manufactured by adding chromium (Cr) and other elements to iron (Fe). It is an alloy steel with improved corrosion resistance, having a chromium content of 10.5% by mass or more and a carbon content of 1.2% by mass or less. The corrosion resistance of stainless steel is achieved by a passive film mainly composed of chromium oxide formed on the metal surface. In addition to iron (Fe) and chromium (Cr), other metallic elements contained in stainless steel include nickel (Ni), molybdenum (Mo), niobium (Nb), titanium (Ti), copper (Cu), aluminum (Al), and manganese (Mn).

[0019] Stainless steel comes in various types, differing in the types and amounts of elements it contains. Stainless steel is primarily classified into martensitic, ferritic, and austenitic types based on its composition and structure. For example, austenitic (non-magnetic) stainless steels include SUS304 (18% Cr, 8% Ni) and SUS316 (18% Cr, 12% Ni, 2.5% Mo), while ferritic (magnetic) stainless steels include SUS430 (18% Cr). Here, % represents mass percent.

[0020] Stainless steel is melted down in electric furnaces and recycled, but there is a need to selectively recover and recycle useful metallic elements from stainless steel. For example, nickel, which is found in austenitic stainless steel, is in high demand in the battery sector, and the EU Battery Regulation stipulates that recycled nickel must be used in a quantity of 6% or more. Therefore, technology for recovering nickel from waste stainless steel is particularly useful as a new metal resource recycling technology.

[0021] The method of the first embodiment is particularly suitable for treating austenitic stainless steel, and more preferably for treating SUS304 or SUS316.

[0022] <Microorganisms> In this specification, the term "microorganism" includes isolated microorganisms and microbial communities consisting of multiple microorganisms. In metal corrosion reactions by microorganisms, the involvement of sulfur-metabolizing microorganisms has been reported in numerous cases, and since sulfur-metabolizing microorganisms accumulate in rock samples originating from the deep sea, the inventors attempted to culture sulfur-metabolizing microorganisms from deep-sea rocks. As a result, they obtained a culture with a microbial community structure that was significantly different from the microbial community structure of the sample before culture. Surprisingly, however, they were able to use this cultured microbial community to promote the dissolution of metals and selectively dissolve specific metal elements from alloys. In the cultured microbial community, more than 50% of the microorganisms had sulfur-oxidizing ability.

[0023] Having sulfur oxidizing ability means that sulfide (S ２－ This means that the microorganisms can oxidize elemental sulfur (S), thiosulfate, polythionic acid, sulfite, and other reduced inorganic sulfur compounds, and the microorganisms having sulfur-oxidizing ability can utilize this oxidation energy as energy for growth. In the method of the first embodiment, microorganisms that grow in a culture medium containing thiosulfate can be preferably used as the microorganisms having sulfur-oxidizing ability. For example, the microorganisms having sulfur-oxidizing ability may be bacteria or archaea. Examples of sulfur-oxidizing bacteria include bacteria belonging to the genera Thiobacillus, Thermthiobacillus, Thiomonas, Thiomicrospira, Beggiatoa, or Halomonas that grow in a culture medium containing thiosulfate. It is also preferable to use the microorganisms having sulfur-oxidizing ability together with other functional microorganisms. In some cases, the dissolution of alloys can be accelerated when microorganisms with multiple functions work together.

[0024] As an example of the microorganisms used in the method of the first embodiment, microorganisms in a culture obtained by culturing marine-derived samples, such as rock samples collected from the seabed where hydrothermal vents are present, in a culture medium containing a sulfur-oxidizing source such as thiosulfate, can be used. For the culture method, please refer to the description in the second embodiment below.

[0025] <Application of Positive Potential> In the method of the first embodiment, by applying a positive potential to the object to be treated, the electrochemical oxidation reaction of metal elements in the object can be promoted, and the dissolution reaction of the object can be advanced. In the method of the first embodiment, the action of microorganisms having sulfur-oxidizing ability suppresses the formation of oxide films on the surface of the object to be treated, such as alloys, and the dissolution of metal elements in the object to be treated is promoted by applying a relatively low voltage. In addition, some sulfur-oxidizing microorganisms have electrochemical activity, and the reaction is promoted by a synergistic effect with the electrochemical metabolism of the microorganisms.

[0026] The application of electric potential can typically be performed using an apparatus that includes at least two electrodes (when the object to be treated is treated as one electrode), a tank containing a medium for immersing the electrodes, a power supply (DC power supply), and wiring connecting the power supply and the electrodes. The wiring should be connected to the conductive parts of the object to be treated. Preferably, the apparatus further includes a reference electrode, a potentiostat, etc. The apparatus may also include a switch for turning the current on and off.

[0027] The object to be processed is placed in the medium together with the cathode electrode as the anode electrode, and a positive potential can be applied to the object by applying a voltage between the two electrodes using a power supply and passing current through them. Platinum or other materials can be used as the cathode electrode.

[0028] The potential applied to the material to be treated can be appropriately selected based on the type of material, the microorganisms used, and the medium, so as to be the potential at which the dissolution of the material progresses. For example, the potential can be slowly increased from the natural immersion potential to select a potential at which the dissolution reaction proceeds at the desired rate. Furthermore, it is preferable to select a potential within a range that does not inhibit the growth of microorganisms. The potential applied to alloys is usually +10 to +2000 mV (vs. Ag / AgCl), and +30 to +1500 mV (vs. Ag / AgCl) is preferred.

[0029] As a medium, any medium that functions as an electrolyte and does not inhibit the survival of microorganisms can be appropriately selected and used. It is also preferable to use a culture medium that can be used to cultivate the microorganisms being used. For example, in a method using microorganisms cultured in a medium containing thiosulfate, it is preferable to use a medium containing thiosulfate as the medium.

[0030] For example, by using a medium containing thiosulfate and applying a potential of approximately 200 mV to +2000 mV, preferably 500 mV to +1500 mV, stainless steel can be dissolved, selectively dissolving iron and nickel, and leaving a high proportion of chromium on the electrode.

[0031] Furthermore, as shown in the examples, by using a medium containing thiosulfate and NaCl and halophilic bacteria, stainless steel can be dissolved by applying a low potential of approximately +30 mV to +200 mV. In addition, iron and chromium can be selectively dissolved, while a high proportion of nickel elements can be left on the electrode.

[0032] <Metal Recovery and Recycling> The method of the first embodiment allows for the recovery of at least a portion of the transition metals contained in the object to be processed. If the object to be processed contains multiple types of transition metals, they can be recovered separately from other types of transition metals. For example, at least a portion of the transition metals constituting an alloy can be dissolved in a medium, and specific transition metals can be recovered from the medium, and it is also possible to recover them as individual elements. Furthermore, in cases where the object to be processed is a structure that includes joints, the joints can be selectively dissolved to dismantle the structure and recover the metal matrix.

[0033] Any known method can be used to recover specific transition metals from a medium; for example, the method described in Surface Technology, Vol. 62, No. 11, 554 (2011) may be used. Alternatively, at least some of the transition metals constituting the object to be treated can be dissolved elementally in the medium, while the remaining specific transition metals can be recovered as metal salts or elemental metals as precipitates (solids) formed in the electrode or medium.

[0034] The dissolved metal can then be further purified by methods such as ion exchange resin extraction, organic solvent extraction, or electric field refining. Therefore, the target metal can be efficiently recovered from materials containing leached metal elements. Furthermore, any remaining metals other than the recovered metal can also be recovered and recycled.

[0035] For example, after recovering nickel from stainless steel (such as SUS304) using the method of the first embodiment, the remaining iron and chromium (which may contain the unrecovered nickel) can be melted in an electric furnace or the like for recycling. In this melting process, raw materials such as ferronickel, ferrochrome, and pig iron may be added.

[0036] <<Second Embodiment>> A second embodiment of the present invention is a method for producing microorganisms that can be used to recover transition metals from a metallic material, and the method includes culturing a sample taken from an environment in which microorganisms having sulfur-oxidizing ability or electrochemical activity are known to be present. The method for producing microorganisms of the second embodiment is preferably a method for producing microorganisms to be used in the method of the first embodiment. By the method for producing microorganisms of the second embodiment, the microorganisms are often obtained as a microbial community, which is a collection of multiple bacteria.

[0037] Examples of samples known to contain microorganisms with sulfur-oxidizing ability or electrochemical activity include rocks containing sulfide minerals such as pyrite.

[0038] Environments where the presence of microorganisms with sulfur oxidation ability or electrochemical activity are known include, for example, the rock surface and interior near hydrothermal vents. It has already been revealed that deep-sea hydrothermal vents act as natural batteries and generate environmental currents (Angew. Chem. Int. Ed. 2013, 52, 10758-10761), and there are also reports of enrichment culture of Thiomicrohabdus bacteria as electrochemically active microorganisms from such environments (The ISME Journal (2023) 17:12-20). Thiomicrohabdus bacteria are known as microorganisms that also have sulfur oxidation ability. In addition, accumulation of electrochemically active bacteria and sulfur-oxidizing bacteria has also been confirmed in metal materials that have been installed on the deep seabed in the northwestern Okinawa Sea and the sea near Aogashima where hydrothermal venting is observed, and where corrosion has been confirmed, indicating that these bacteria are widely distributed. Accumulation of similar microorganisms has also been confirmed on corroded stainless steel in Sagami Bay and the Yokosuka coast, and it is thought that accumulation can be easily achieved by corroding stainless steel even in shallow seas. In addition, the presence of microorganisms can be confirmed by immersing a test piece of the treatment target and checking for the presence or absence of corrosion. Examples of samples include rocks and seawater present on the seabed. The rock may be crushed into fine pieces and immersed in a culture medium. Alternatively, the corroded portion of a test piece of the treatment target in which the presence of microorganisms has been confirmed as described above may be used as a sample.

[0039] In the method according to the second embodiment, the culture is carried out in at least sulfide (S ２－ ), elemental sulfur (S), thiosulfuric acid, polythionic acid, sulfurous acid and other reduced inorganic sulfur compounds. It is preferable to carry out the culture in a medium containing the above, and more preferable to carry out the culture in a medium containing thiosulfate. For the medium, reference can be made to the description in the first embodiment.

[0040] As a medium used for culturing microorganisms, a liquid medium can be used. The medium contains at least an oxidizable sulfur source, for example, sulfide (S ２－ ), elemental sulfur (S), thiosulfuric acid, polythionic acid, sulfurous acid and other reduced inorganic sulfur compounds. It is preferable that the medium contains the above, and more preferable that it contains thiosulfate. As the thiosulfate, for example, sodium thiosulfate (Na ２S ２ O ３ ), potassium thiosulfate (K ２ S ２ O ３ ), calcium thiosulfate (CaS ２ O ３ ), etc.; sodium thiosulfate or potassium thiosulfate is preferred, and sodium thiosulfate is particularly preferred.

[0041] The medium may or may not contain organic carbon sources such as saccharides, alcohols, and fatty acids. On the other hand, the medium contains an appropriate amount of nitrogen source, inorganic salts and other nutrients. As the nitrogen source, it is preferable to use ammonia, ammonium salts such as ammonium nitrate, ammonium sulfate, and ammonium chloride, nitrates such as sodium nitrate, and inorganic nitrogen sources such as urea.

[0042] As the inorganic substances, examples include phosphates, hydrochlorides, sulfates, nitrates, acetates, and halides such as chlorides of magnesium, calcium, sodium, potassium, manganese, copper, nickel, iron, zinc, and the like. Said inorganic substances are appropriately selected in consideration of the assimilability by the microorganism to be pre-cultured. Furthermore, one or more of said inorganic substances can be selected and used.

[0043] As the medium, any other medium known as a medium for growing and proliferating sulfur-oxidizing bacteria can be used.

[0044] As an example of the medium, a medium having the thiosulfate-containing inorganic salt medium composition shown in Table 1 can be mentioned. As shown in the Examples, by culturing microorganisms in a deep-sea-derived sample in this medium, a microbial community mainly containing bacteria of the genus *Thiobacillus* and bacteria of the family *Thiomicrospiraceae* can be obtained. Using this microbial community, it was possible to selectively dissolve iron and nickel from stainless steel (SUS316) and recover chromium as chromium oxide.

[0045] Another example of a culture medium is one to which NaCl is added at a concentration of 1.2 to 15% by mass (0.2 to 2.5 M), preferably 1.2 to 2.9% by mass, to the thiosulfate-containing inorganic salt medium composition shown in Table 1. As shown in the examples, by culturing microorganisms from deep-sea samples in this medium, a microbial community mainly consisting of Halomonas bacteria was obtained. Using this microbial community, it was possible to selectively dissolve iron and chromium from stainless steel (SUS316) and recover a metal containing a high proportion of nickel at the electrode.

[0046] Microbial culture can be carried out under aerobic or anaerobic conditions, but aerobic conditions are preferred. Microbial culture can be made aerobic by shaking or aeration stirring. Furthermore, the culture conditions can be appropriately selected depending on the composition of the culture medium and the culture method, and are not particularly limited as long as the desired microorganisms can grow, and can be appropriately selected according to the type of microorganism being cultured.

[0047] The upper limit of the pH of the culture medium used for cultivation is not particularly limited, but is usually 10.0 or less, preferably 8.0 or less. The lower limit of the pH of the above culture medium is not particularly limited, but is preferably 0.8 or higher.

[0048] The lower limit of the culture temperature is not particularly limited, but is usually 10°C or higher, preferably 20°C or higher. The upper limit of the culture temperature is not particularly limited, but is preferably 40°C or lower, for example.

[0049] The culture time for microorganisms is not particularly limited and can be set arbitrarily depending on the type of microorganism being cultured and the culture conditions. For example, it is 12 to 720 hours, preferably 24 to 168 hours. It is preferable to use microorganisms that have reached the logarithmic growth stage, in which they are in a physiologically favorable state, in the method of the first embodiment.

[0050] In the method according to the second embodiment, it is preferable to confirm whether the culture obtained by the above culture or the microorganisms in the culture have the activity to promote the dissolution of the target substance. For example, the culture can be added to a medium in which the alloy to be treated is immersed, and an electric potential can be applied to the alloy to confirm whether at least a part of the alloy has dissolved, and the culture in which dissolution has occurred can be selected. Here, it is preferable to use the culture medium used for cultivation as the medium. For the application of a positive electric potential, etc., refer to the explanation in the first embodiment.

[0051] <<Other Embodiments>> By applying the method for recovering transition metals from the object to be processed in the first embodiment, structures containing transition metals can be dismantled. Specifically, by applying an electrolytic gel containing a culture medium containing microorganisms to a part of the structure (such as a stainless steel pipe), and then applying a voltage to the structure, specific transition metals are dissolved only in the aforementioned area. This area can then be used as a cutting point, allowing the structure to be dismantled with less energy. Furthermore, by applying the method for recovering transition metals from the object to be processed, it is also possible to perform microfabrication in combination with masking.

[0052] The present invention will be described in more detail below with reference to examples. The materials, reagents, amounts and proportions of substances, and procedures shown in the following examples can be modified as appropriate, as long as they do not depart from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the following examples.

[0053] (1) Culture Test Rock samples collected from the Higashi-Aogashima Seamount Caldera (approximately 700 m deep) were crushed to a size of 1 cm square or less using a chisel and hammer. They were transferred to Erlenmeyer flasks and cultured for 30 days at 30°C with shaking using a thiosulfate-containing inorganic salt medium with the composition shown in Table 1. Culture was also carried out in the same manner using a thiosulfate-containing artificial seawater medium (a medium prepared by adding sodium chloride to the thiosulfate-containing inorganic salt medium with the composition shown in Table 1 to a concentration of 2.7% [w / v]) instead of the thiosulfate-containing inorganic salt medium.

[0054]

[0055] (2) Analysis of the microbial community structure of the accumulated cultures DNA was extracted from the culture medium after culturing in each medium. The culture medium was filtered using a membrane filter with a pore size of 0.22 μm to collect microbial cells, and the membrane filter was cut into 1-2 mm squares and placed in a cell disruption tube. A commercially available kit (FAST DNA SPIN Kit for SOIL) was used for DNA extraction. Using the extracted DNA as a template, the 16S rRNA gene (16S rDNA) was partially amplified by PCR using the U530F (GTGCCAGCCMGCGCGG: SEQ ID NO: 1) / U907R (CCGTCAATTCCMTTTRAGTTT: SEQ ID NO: 2) primer set targeting bacteria and archaea, and the base sequence was deciphered using MiSeq Reagent Kit v3 and the next-generation sequencer MiSeq. The obtained data were analyzed using the QIIME2 pipeline to estimate the relative amounts of microbial species contained in each sample. The results of comparing the microbial community structures of rock samples and cultures are shown in Figures 1 and 2. For the microbial community structure of the rock samples, the rock was crushed into pieces smaller than 5 mm square using a chisel and hammer and placed in cell disruption tubes. Subsequently, DNA extraction was performed in the same manner, PCR amplification was carried out, the base sequence was decoded using the next-generation sequencer MiSeq, and the data was analyzed using the QIIME2 pipeline.

[0056] In culture 229R01-30 on a thiosulfate-containing inorganic salt medium, bacteria of the genus Thiobacillus and the family Thiomicrospiraceae were accumulated from below the detection limit to 54.7% and 29.1%, respectively (Figure 1). Both microbial species have been reported as sulfur-oxidizing bacteria and are thought to be sulfur-oxidizing bacteria that grow using thiosulfate as an energy source.

[0057] In cultures prepared in thiosulfate-containing artificial seawater, Halomonas bacteria accumulated in culture 230R05-30Na from below the detection limit to 90.1% (Figure 2). Some species of Halomonas bacteria have been reported to possess thiosulfate oxidation ability, and the microbial species accumulated here are also thought to be Halomonas bacteria with thiosulfate oxidation ability.

[0058] (3) Electrochemical Test Two SUS316 stainless steel test pieces measuring 25 mm x 20 mm x 2 mm were prepared, and a welded test piece was prepared by welding the short sides together. A coated copper wire was soldered to the upper end of this test piece, and the entire piece was covered with insulating silicone resin, leaving a 20 mm x 10 mm exposed surface so that the weld line was positioned in the center, to serve as the stainless steel test piece electrode. 1 L of thiosulfate-containing inorganic salt medium or thiosulfate-containing artificial seawater medium was placed in a 150 mm x 150 mm x 100 mm (H) acrylic container, and a three-electrode electrochemical test system was prepared with the stainless steel test piece electrode as the working electrode, silver / silver chloride (3 M KCl solution) as the reference electrode, and a platinum electrode as the counter electrode. The stainless steel test piece electrode was placed horizontally at the bottom, and time-lapse photography was performed from above using a digital microscope (Keyence VHX-8000) to investigate the electrochemical behavior and the timing of corrosion initiation.

[0059] Stainless steel test electrode specimens were immersed in a thiosulfate-containing inorganic salt culture medium. After the potential stabilized, the potential was maintained at 900 mV, and the amount of electricity was measured. In the case of microbial cultures, the aggregate culture 229R01-30 was used. Because this constant potential maintenance experiment was conducted in a freshwater culture medium where stainless steel corrosion is less likely to occur, no significant active dissolution occurred even after 24 hours of potential maintenance under the microorganism-free condition. However, under the microorganism-included condition, the current value increased rapidly after about 8 hours of immersion, and active dissolution progressed rapidly (Figure 3). After 20 hours from the start of the experiment, the solution became so turbid that the electrode could not be visually inspected, so the potential maintenance was released, and the electrode was removed for observation. Under the microorganism-free condition, brown oxide formation was observed on the exposed parts, but active dissolution had not occurred. On the other hand, under the microorganism-included condition, significant active dissolution occurred, and a considerable amount of corrosion products were formed overall (Figure 4). When electrodes on which corrosion products had formed were washed with a 50% (v / v) hydrochloric acid solution containing 0.7% (w / v) hexamethylenetetramine, the weld lines remained relatively intact.

[0060] A similar experiment was conducted using an artificial seawater culture medium containing thiosulfate. The culture medium used in this experiment was the microbial culture 230R05-30Na. When the potential was slowly increased from the natural immersion potential, dissolution of the stainless steel was observed at around +30 mV (Figure 5). Subsequently, when the applied voltage was increased to +200 mV, the dissolution reaction continued, and the entire exposed surface became highly corroded (Figure 6).

[0061] (4) Analysis of corrosion products After the electrochemical test, a significant metal dissolution reaction was observed in the microbial area. Therefore, elemental analysis was performed on the corrosion products formed on the surface of the stainless steel test specimen electrodes using a laser-induced breakdown spectroscopy (LIBS) device mounted on a digital microscope. Multiple locations were selected, including healthy parts where no corrosion reaction had occurred and corroded areas, and 9 to 25 spots were analyzed at each location.

[0062] Samples corroded in a thiosulfate-containing inorganic salt medium in the presence of aggregate culture 229R01-30 showed a metallic elemental composition (Fe [53.4%–70.2%, Cr [14.9%–25.8%], Ni [7.8%–13.6%]) nearly identical to that of stainless steel near the weld line where corrosion had not occurred (Figure 7). On the other hand, in the corroded areas, chromium oxide (Cr [5.8%–20.2%], O [39.3%–86.4%]) was detected where white corrosion products had formed on the surface (Figure 8).

[0063] Samples corroded in thiosulfate-containing artificial seawater medium in the presence of the aggregate culture 230R05-30Na formed black corrosion products. On the other hand, no dissolution reaction occurred in the healthy sections covered with tape, and components corresponding to the composition of stainless steel were detected (Figure 9). The component composition of the dissolved areas was detected as 100% nickel or nickel compounds (O [0-85.9%], Fe [0-38.4%], H [0-3.5%]). 100% nickel was detected in 88 out of 108 spots analyzed (Figure 10). The overall average value was also high at Ni (94.4%), indicating that iron and chromium were selectively dissolved, while nickel specifically remained, compared with the healthy sections.

[0064] Energy-dispersive X-ray fluorescence analysis (mapping-ED-XRF) was performed on the same sample (a sample that was corroded in thiosulfate-containing artificial seawater medium in the presence of the accumulated culture 230R05-30Na, forming a black corrosion product). A mapping-ED-XRF SEA6000vx (Hitachi High-Tech Science) was used, and the sample was placed on the stage without pretreatment and scanned at a tube voltage of 50kV and a resolution of 50μm / pixel. The two-dimensional distribution of Ni is imaged in Figure 11.

[0065] Similar to the results obtained from LIBS point analysis, it was confirmed that Ni was more concentrated in the dissolved residual area compared to the coated (sound) area and the welded area. On the other hand, although not shown in the figure, the two-dimensional distribution of Cr and Fe was imaged, and in both cases, it was confirmed that Cr and Fe were more concentrated in the dissolved residual area compared to the coated (sound) area and the welded area. This result indicates that the LIBS results are not localized but are a phenomenon occurring throughout the entire dissolved residual area.

[0066] Furthermore, the three-dimensional structure of the same sample was analyzed. The analysis was performed using a FIB-SEM (Helios G4 UX, Thermo Fisher Scientific) equipped with an EDS (Octane elite Super (C5), AMETEK). The sample was fixed to an aluminum sample stage with carbon double-sided tape (Oken Shoji), and observation and surface EDS (energy-dispersive X-ray spectroscopy) analysis were performed without vapor deposition. Subsequently, for areas where metal dissolution was confirmed by EDS analysis, the sample was tilted at 52° to cut vertically, and an area of ​​approximately 300 μm × 100 μm was cut with a Ga ion beam (30 kV, 65 nA) (Figure 12). The cut sample was then tilted at 30° and observed with an SEM (scanning electron microscope) (Figure 13). SEM observation was performed at an accelerating voltage of 2 kV, and EDS analysis at 20 kV. The outermost layer showed a finely particulated structure, while the structure became more organized with larger particle sizes as we went deeper. This indicates that the dissolution reaction was not a phenomenon occurring only at the very surface, but rather proceeded in three dimensions.

[0067] This invention enables the recovery of transition metals such as nickel from metal-containing structures such as stainless steel with a low environmental impact, thereby promoting metal recycling. The method of selectively dissolving specific metals can also be applied to the microfabrication and dismantling of metal-containing structures.

Claims

1. A method for recovering at least a portion of transition metals contained in a material to be treated, comprising applying a positive potential to the material in a medium in the presence of a microorganism having sulfur-oxidizing ability, thereby dissolving at least a portion of the material to be treated in the medium.

2. The method according to claim 1, wherein the material to be processed includes iron and nickel.

3. The method according to claim 2, wherein the material to be processed is stainless steel and nickel is recovered.

4. The method according to claim 3, wherein the medium contains a thiosulfate.

5. The method according to claim 4, wherein the microorganism is a culture obtained by culturing a marine-derived sample in a medium containing thiosulfate.

6. The method according to any one of claims 3 to 5, wherein the microorganism includes bacteria of the genus Halomonas, and nickel is recovered individually.

7. The method according to claim 6, wherein the medium comprises a thiosulfate and 1.2% to 15% by mass of NaCl.

8. The method according to any one of claims 3 to 5, wherein the microorganism comprises at least one selected from the genus Thiobacillus and the genus Thiomicrospira, and nickel is dissolved in the medium as at least a portion of the object to be treated.

9. A method for producing microorganisms for use in the method according to any one of claims 1 to 5, comprising culturing a rock sample collected from the seabed where hydrothermal vents are present or a rock sample containing sulfide minerals in a culture medium containing thiosulfate.

10. The method according to claim 9, comprising immersing the object to be treated in a culture medium containing the culture obtained in the culture and a thiosulfate, and applying a positive potential to select a culture in which at least a portion of the object to be treated is confirmed to be dissolved.

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