Method for decontamination of radionuclide contaminated soil

Abstract

The present invention relates to a method for decontaminating soil contaminated with radionuclides, and more specifically, to a high-efficiency method for decontaminating soil contaminated with radionuclides that can significantly improve the removal efficiency of cesium by preventing the re-adsorption of cesium within clay minerals of soil contaminated with radionuclides leaked from nuclear-related facilities, while simultaneously simplifying the decontamination process to improve process time and economic efficiency.

Description

Technology Field

[0001] The present invention relates to a method for decontaminating soil contaminated with radionuclides, and more specifically, to a high-efficiency method for decontaminating soil contaminated with radionuclides that can significantly improve the removal efficiency of cesium by preventing the re-adsorption of cesium during the decontamination process of soil contaminated with radionuclides leaked from nuclear-related facilities, while simultaneously simplifying the decontamination process to improve process time and economic efficiency. Background Technology

[0003] Radioactive contamination of the soil within the site can occur due to leaks during the operation of nuclear facilities, such as through pipe spills. It has been found that the decommissioning of overseas commercial nuclear power plants generates large quantities of soil waste contaminated with cesium (Cs-137), strontium (Sr-90), cobalt (Co-60), and tritium (H-3). In the United States, 52,800 tons of soil waste were generated at the Connecticut Yankee, while at the Main Yankee, waste accounted for 10% of the total waste. In the case of commercial nuclear power plant decommissioning in the United States, environmental restoration costs were reported to average $60 million per unit, accounting for an average of 10% of the total decommissioning costs. Globally, approximately 220 commercial nuclear power plants are expected to be decommissioned by 2030, and the market for the restoration of radioactive decommissioning sites following nuclear plant decommissioning is expected to expand significantly. In Korea as well, Kori Unit 1 and Wolsong Unit 1 have been permanently shut down and are scheduled to proceed with decommissioning in the future.

[0004] In particular, among the radionuclides that cause soil contamination, radioactive cesium ( 137 Cesium (Cs) is produced by the nuclear fission of uranium and plutonium, and with a half-life of 30.2 years, 1 g of cesium emits approximately 3.22 terabecquerels (TBq) of radioactivity and is a radionuclide that must be prioritized for removal from radioactive contaminated soil due to its high heat generation characteristics.

[0005] However, conventional methods for removing cesium from radioactive contaminated soil have limitations for the following reasons.

[0007] First, among radionuclides, cesium is reported to be present mainly within 5 cm of the soil surface, which presents a problem in that it is not easily removed due to selectivity for clay minerals and strong adsorption. Specifically, the adsorption sites within the clay include outer-sphere bonding of positively charged cesium to negatively charged planar sites via electrostatic adsorption, and inner-sphere bonding to internal sites (interlayer sites or frayed edge sites, (FES)) that selectively adsorb cesium. In particular, in the case of inner-sphere bonding, it is difficult to desorb due to strong binding. Furthermore, cesium fixed to FES can diffuse into deeper internal spaces over time through local exchange with adjacent cations, making desorption even more difficult.

[0009] Secondly, although various studies have been reported to address the issues caused by the selectivity and strong adsorption of cesium on clay minerals and to improve cesium desorption efficiency, there is a problem in achieving the desired desorption efficiency because cesium desorbed from clay tends to be re-adsorbed at thermodynamically stable sites. Specifically, among previously reported studies, H2O2 treatment using divalent cation Mg can improve cesium desorption by combining with the H2O2 decomposition reaction to facilitate interlayer expansion by hydrated divalent cations. Similarly, studies using ultrasound and high temperatures can expand the interlayers of clay minerals, allowing divalent Mg ions to access trapped cesium and improve the removal rate. However, these methods face the problem of being unable to desorb cesium that has been re-adsorbed at sites with high affinity for cesium, such as internal regions within the clay minerals (interlayer sites and FES), making it difficult to achieve the desired removal rate.

[0011] Third, in decontamination processes where adsorbents coexist, it is necessary to separate the adsorbent from the soil suspension after the washing process; however, existing adsorbents, such as powders, are micro-sized, making it difficult to separate them after the cesium adsorption process. In other words, even if cesium is separated from the contaminated soil, additional processes are required to separate the adsorbent present in the suspension, which becomes a factor that reduces process efficiency and economic feasibility.

[0013] Accordingly, there is an urgent need for research on a highly efficient decontamination method for radioactive nuclide-contaminated soil that can significantly improve the removal efficiency of cesium from soil contaminated with radionuclides at nuclear-related facilities, address conventional problems by preventing the re-adsorption of cesium within clay minerals, and enable the easy separation of the adsorbent that has adsorbed cesium. Prior art literature

[0015] Republic of Korea Published Patent Application 10-2015-0137201 (December 09, 2015) The problem to be solved

[0016] The present invention has been devised to overcome the aforementioned problems. The objective of the present invention is to provide a method for decontaminating radioactive contaminated soil that can improve the washing efficiency of radioactive contaminated soil by introducing an adsorbent with high selectivity for cesium to adsorb cesium desorbed by acid washing with high efficiency and prevent the re-adsorption of the desorbed cesium onto the contaminated soil, thereby reducing the amount of radioactive waste generated in the decontamination process of radioactive contaminated soil and improving economic efficiency.

[0017] In addition, another problem that the present invention aims to solve is to provide a method for decontaminating soil contaminated with radionuclides, which facilitates the separation of the adsorbent through sieve separation or columns by using a bead-type adsorbent, simplifies the process by allowing the treatment of cesium-contaminated clay and the treatment of cesium in the clay washing solution to be performed simultaneously within a single system, and reduces process operating costs by consuming less energy. means of solving the problem

[0019] To solve the above-mentioned problem, the present invention provides a method for decontaminating soil contaminated with radioactive nuclides, comprising: a first step of introducing an acid and a PAN (Polyacrylonitrile)-based metal ferrocyanide adsorbent into the soil contaminated with radioactive nuclides to desorb the radioactive nuclides from the contaminated soil and adsorbing the desorbed radioactive nuclides onto the PAN-based metal ferrocyanide adsorbent; and a second step of separating the PAN-based metal ferrocyanide on which the radioactive nuclides are adsorbed.

[0020] In addition, according to one embodiment of the present invention, the radioactive nuclide-contaminated soil may be characterized by comprising a clay mineral having high selectivity and strong binding power for radioactive cesium.

[0021] In addition, the first step above may be characterized by washing with an inorganic acid of 0.1 to 1.5 M.

[0022] In addition, the first step above may be characterized by being performed at 20 to 80 ℃.

[0023] In addition, the PAN-based metal ferrocyanide adsorbent of the first step may be characterized by being prepared by mixing a metal ferrocyanide selected from the group consisting of Ni, Co, Cu, and Ni, which has excellent adsorption capacity for PAN-based compounds and cesium, in a weight ratio of 1:0.5 to 5.

[0024] In addition, the first step above may be characterized by adding a PAN-based metal ferrocyanide adsorbent to the radioactive nuclide-contaminated soil in a weight ratio of 1:0.001 to 0.2.

[0025] In addition, the PAN-based metal ferrocyanide adsorbent of the first step may be characterized by preventing cesium that has been desorbed by washing with acid from being re-adsorbed onto clay minerals.

[0026] In addition, the PAN-based metal ferrocyanide adsorbent of the second stage may be characterized as being spherical in size of 0.5 to 5 mm.

[0027] In addition, the present invention provides a PAN-based metal ferrocyanide adsorbent for decontaminating radioactive nuclide-contaminated soil, having a bead form in which a PAN-based compound and a metal ferrocyanide are mixed in a weight ratio of 1:0.5 to 5, and having a removal rate of cesium of 85% or more.

[0028] In addition, according to one embodiment of the present invention, the PAN-based compound may be characterized as being selected from the group consisting of Ni, Co, Cu, and Ni. Effects of the invention

[0030] The present invention improves the efficiency of clay washing by introducing a PAN-based metal ferrocyanide adsorbent with high selectivity for cesium, thereby adsorbing cesium desorbed by acid washing and preventing re-adsorption onto contaminated soil. This allows for a reduction in radioactive waste generated during the soil washing process, thereby improving economic feasibility. Furthermore, since the treatment of cesium-contaminated clay and the treatment of cesium within the clay washing solution are carried out simultaneously within a single system, the process can be simplified, and the low energy consumption can significantly reduce process operating costs. Brief explanation of the drawing

[0032] FIG. 1 is a schematic diagram showing a method for decontaminating radioactive nuclide-contaminated soil using a PAN-based metal ferrocyanide adsorbent according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a method for decontaminating radioactive nuclide-contaminated soil using a PAN-based metal ferrocyanide adsorbent according to another embodiment of the present invention. Figures 3 and 4 are graphs showing the adsorption rate and adsorption amount according to the weight ratio of PAN-based metal ferrocyanide adsorbent in distilled water and acid according to one embodiment of the present invention. Figure 5 is a graph showing the evaluation of cesium desorption of a PAN-based metal ferrocyanide adsorbent under temperature conditions according to one embodiment of the present invention. Figure 6 is a graph showing the evaluation of cesium desorption from clay (HBT) according to the amount of PAN-based metal ferrocyanide adsorbent according to one embodiment of the present invention. FIG. 7 is a schematic diagram showing the separation step of a PAN-based metal ferrocyanide adsorbent according to one embodiment of the present invention. Specific details for implementing the invention

[0033] The embodiments of the present invention are described below in detail so that those skilled in the art can easily implement the invention. The present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0035] As mentioned above, conventional decontamination methods for soil contaminated with radionuclides leaked from nuclear-related facilities face challenges in efficiently removing cesium from contaminated soil, particularly due to issues such as low decontamination efficiency for cesium, re-adsorption of clay minerals, and excessive processing time and costs during the adsorbent separation process.

[0036] Accordingly, the present invention seeks to solve the aforementioned problem by providing a method for decontaminating soil contaminated with radionuclides, comprising: a first step of introducing an acid and a PAN (Polyacrylonitrile)-based metal ferrocyanide adsorbent into the soil contaminated with radionuclides to desorb the radionuclides from the contaminated soil and adsorbing the desorbed radionuclides onto the PAN-based metal ferrocyanide adsorbent; and a second step of separating the PAN-based metal ferrocyanide on which the radionuclides are adsorbed.

[0037] Through this, the present invention can significantly improve the removal efficiency of cesium by preventing the re-adsorption of cesium within clay minerals of soil contaminated with radionuclides leaked from nuclear-related facilities, while simultaneously simplifying the decontamination process to improve process time and economic efficiency.

[0039] A method for decontaminating soil contaminated with radionuclides according to the present invention is described below.

[0040] The first step of the method for decontaminating soil contaminated with radionuclides according to the present invention is to introduce an acid and a PAN (Polyacrylonitrile)-based metal ferrocyanide adsorbent into the soil contaminated with radionuclides to desorb the radionuclides from the contaminated soil and to adsorb the desorbed radionuclides onto the PAN-based metal ferrocyanide adsorbent. Generally, in the case of radioactive cesium, due to its high selectivity and strong binding to clay minerals, high contamination may still be present in the clay portion even after washing the contaminated soil. Furthermore, since the adsorption of cesium onto clay minerals in soil contaminated with radionuclides is a reversible reaction, cesium desorbed by acid washing may be re-adsorbed onto the clay minerals. Such issues regarding the high selectivity, strong binding, and re-adsorption of radioactive cesium onto clay minerals are causes that significantly reduce the removal efficiency of cesium during the decontamination process.

[0042] Accordingly, the present invention solves the problem of cesium re-adsorption onto clay minerals by introducing an acid and a PAN-based metal ferrocyanide adsorbent to effectively remove cesium in the first step and prevent the re-adsorption of cesium desorbed by acid washing, thereby desorbing radionuclides, particularly cesium, from contaminated soil and simultaneously adsorbing the desorbed radionuclides onto the PAN-based metal ferrocyanide adsorbent. That is, when soil contaminated with cesium containing clay minerals is washed with acid, cesium is desorbed from the soil and exists in a suspension state. At this time, if an adsorbent is introduced into the suspension, the desorbed cesium is adsorbed by the adsorbent, but some of it may be re-adsorbed back onto the clay minerals of the soil. In other words, some of the cesium desorbed from the soil by washing with acid may be re-adsorbed to the outer sphere through electrostatic bonding on the surface of negatively charged planar sites within the clay. In addition, cesium that has been washed away from soil and desorbed can be re-adsorbed through inner-sphere bonding to internal sites (interlayer sites or frayed edge sites, (FES)) that selectively adsorb it. In particular, in this case, there is a problem that it is difficult to desorb it again due to the stronger bonding between clay and cesium. Furthermore, cesium fixed to FES can diffuse into deeper internal spaces over time through local exchange with adjacent cations, making it difficult for PAN-based metal ferrocyanide adsorbents to access and thus making it more difficult to desorb cesium.

[0043] The present invention can improve the cesium removal efficiency by simultaneously introducing an acid and a PAN-based metal ferrocyanide adsorbent through the first step to improve the problem of re-adsorption of cesium on clay minerals described above.

[0045] More specifically, referring to FIG. 1, in accordance with one embodiment of the present invention, when an aqueous solution containing an acid and a PAN-based metal ferrocyanide adsorbent is introduced into soil contaminated with cesium in the first step, the PAN-based metal ferrocyanide adsorbent can adsorb cesium that is washed away and desorbed by the acid, and at the same time, prevent the cesium that is washed away and desorbed by the acid from being re-adsorbed onto clay minerals.

[0046] In addition, according to another embodiment of the present invention, as shown in FIG. 2, a washing solution containing acid is injected through a column process to perform cesium desorption from contaminated soil containing clay minerals (first column), and the cesium desorbed in the washing solution can be removed through a PAN-based metal ferrocyanide adsorbent (second column). In this case, sieving or separating the adsorbent through a column is also easy, and since the treatment of cesium-contaminated clay and the treatment of cesium in the clay washing solution are performed simultaneously within a single system, the process can be simplified and process operating costs can be reduced due to low energy consumption.

[0047] Referring to Table 1, it can be seen that the desorption efficiency of cesium through the first step according to one embodiment of the present invention is significantly improved compared to the conventional method. More specifically, to verify the desorption effect of the first step according to the present invention for radioactive soil decontamination, an adsorbent in the form of PAN-based metal ferrocyanide beads corresponding to 10% of the total weight of the contaminated soil and 0.5 M HNO3 was added, and the desorption efficiency was evaluated on clay (HBT) contaminated with the radionuclide cesium-137 under conditions of 60°C, and the radioactivity of the initially contaminated clay was 209 Bq / g, which was less than 0.1 ppb. At this time, when only acid washing was performed without using the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention, the radioactivity decreased from 209 Bq / g to 70 Bq / g and the removal efficiency was 66.5%. However, when the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention was added simultaneously with acid washing, the radioactivity of the contaminated clay decreased from 209 Bq / g to 27 Bq / g and the removal efficiency was significantly improved to 87.1%. Through these results, it can be seen that desorption based on the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention can significantly improve decontamination efficiency by preventing the re-adsorption of radioactive cesium in contaminated clay. Furthermore, referring to Table 2, it can be seen that the desorption rate of the first step according to the present invention is significantly superior compared to the conventional clay desorption technology using acidic solutions and cations.

[0049] To this end, the above-mentioned PAN-based metal ferrocyanide adsorbent can be formed by mixing a PAN-based compound and a metal ferrocyanide selected from the group consisting of Ni, Co, Cu, and Ni, which have excellent adsorption capacity for cesium, so that the PAN-based compound is stable in a strong acid solution along with other competing ions in radioactive nuclide-contaminated soil or washing solution and can selectively adsorb cesium, and the PAN-based compound and the metal ferrocyanide can be mixed in a weight ratio of 1:0.5 to 5. At this time, if the above-mentioned PAN-based compound and the metal ferrocyanide are mixed in a weight ratio of less than 1:0.5, there may be a problem of reduced adsorption rate, and if the above-mentioned PAN-based compound and the metal ferrocyanide are mixed in a weight ratio exceeding 1:5, there may be a problem of difficulty in forming the PAN adsorbent. That is, by referring to FIG. 3, the adsorption rate and adsorption amount according to the weight ratio of the PAN-based compound and the metal ferrocyanide can be determined, and based on this, the weight ratio of the PAN-based compound and the metal ferrocyanide can be appropriately changed according to the amount, type, and amount of cesium of the target contaminated soil.

[0050] Meanwhile, the method of preparing a PAN-based metal ferrocyanide adsorbent by mixing the above-mentioned PAN-based compound and metal ferrocyanide can be prepared using a known conventional manufacturing method that satisfies the weight ratio described above and is consistent with the purpose of the present invention. For example, a surfactant can be added to a metal ferrocyanide precursor mixed solution and stirred at 30 to 70°C for 0.5 to 2 hours, and then a PAN-based compound powder can be added to the mixed solution and stirred at 30 to 70°C for 3 to 7 hours. Afterward, the mixed solution can be prepared into spherical beads with a diameter of 2 mm by dropping it into distilled water, and the finished beads can be washed and dried several times to finally prepare the PAN-based metal ferrocyanide adsorbent according to the present invention.

[0052] In addition, the acid simultaneously introduced with the PAN-based metal ferrocyanide adsorbent in the first step may be an inorganic acid with a concentration of 0.1 to 1.5 M. In this case, if the inorganic acid simultaneously introduced with the PAN-based metal ferrocyanide adsorbent is HNO3 with a concentration of less than 0.1 M, there may be a problem with reduced desorption efficiency; furthermore, if the inorganic acid simultaneously introduced with the PAN-based metal ferrocyanide adsorbent is HNO3 with a concentration exceeding 1.5 M, there may be a problem with reduced adsorption capacity due to damage to the PAN-based adsorbent. In this case, the inorganic acid may be a known conventional inorganic acid suitable for the purpose of the present invention, for example, HNO3, H2SO4 and HCl can be used.

[0053] Referring to Figures 3 and 4, it can be seen that the adsorption capacity of the metal ferrocyanide powder and the PAN-based metal ferrocyanide adsorbent is similar in DI water, but the adsorption capacity of the PAN-based metal ferrocyanide adsorbent is significantly superior under acidic conditions where the PAN-based metal ferrocyanide adsorbent is simultaneously introduced in the first step. This indicates that while the metal ferrocyanide powder is damaged and its adsorption capacity decreases under acidic conditions, the PAN-based metal ferrocyanide adsorbent's adsorption capacity is not affected because PAN has resistance to acid. In other words, since the primary objective of the present invention is to prevent the re-adsorption of cesium by introducing an adsorbent under acidic conditions, it can be seen that acidic conditions above a certain concentration and the PAN-based metal ferrocyanide adsorbent are required simultaneously to improve the decontamination efficiency against cesium.

[0055] In addition, the first step can be performed at 20 to 80 ℃. More specifically, referring to FIG. 5, it can be seen that when treated at a high temperature of 60 ℃, the cesium desorption efficiency increases from 30.3% to 51.7% as the HNO3 concentration increases from 0.1 to 1.0 M. Through this, it can be seen that cesium desorption increases significantly as the HNO3 concentration and treatment temperature increase, and that the mobility of cesium in clay minerals can be improved by increasing the temperature and HNO3 concentration. However, if the temperature exceeds 80 ℃, additional devices such as pressure control are required under washing conditions, which is not desirable in terms of economic feasibility or process operation.

[0056] In addition, the PAN-based metal ferrocyanide adsorbent introduced into the radioactive nuclide-contaminated soil in the first step may be introduced in a weight ratio of 1:0.001 to 0.2 relative to the weight of the radioactive nuclide-contaminated soil. More specifically, referring to FIG. 7, it can be seen that cesium desorption increases as the amount of the introduced PAN-based metal ferrocyanide adsorbent increases, and according to one embodiment of the present invention, the highest cesium desorption rate of 73.3% is observed with a weight of 10% of NiFC-PAN beads. However, if the amount of the introduced PAN-based metal ferrocyanide adsorbent exceeds a weight ratio of 0.2 relative to the weight of the radioactive nuclide-contaminated soil, there may be a problem of increased waste volume because the NiFC-PAN beads are classified as radioactive waste after adsorption.

[0059] Meanwhile, in decontamination processes where adsorbents coexist, it is necessary to separate the adsorbent from the soil suspension after the washing process; however, there was a problem in that the adsorbents used in the past, such as powders, were micro-sized and difficult to separate after the cesium adsorption process. In other words, even if cesium is separated from the contaminated soil, additional processes are required to separate the adsorbents present in the suspension, which becomes a factor that reduces process efficiency and economic feasibility.

[0060] Accordingly, the present invention solves the aforementioned problem by performing a second step of separating PAN-based metal ferrocyanides adsorbed with radionuclides through the first step. More specifically, referring to FIG. 8, it can be seen that the PAN-based metal ferrocyanide adsorbent according to the present invention is capable of separation through a sieve because its particles are larger than those of hydrobiotite (clay). To this end, the PAN-based metal ferrocyanide adsorbent in the second step may be spherical in size of 0.5 to 5 mm. In this case, if the size of the PAN-based metal ferrocyanide adsorbent is less than 0.5 mm, there may be a problem with difficulty in separation, and if the size of the PAN-based metal ferrocyanide adsorbent exceeds 5 mm, there may be a problem with reduced adsorption capacity. Meanwhile, according to a preferred embodiment of the present invention, if it is spherical in size, it may be more advantageous for the separation process in the second step.

[0061] The PAN-based metal ferrocyanide adsorbent according to the present invention, manufactured through this process, has a bead form in which a PAN-based compound and a metal ferrocyanide are mixed in a weight ratio of 1:0.5 to 5, and can exhibit a removal rate of 85% or more for cesium according to the following measurement method.

[0062] [measurement method]

[0063] For radioactive clay (HBT) with a radioactivity of 209 Bq / g and less than 0.1 ppb, the removal rate of cesium-137 is measured by adding an adsorbent at a weight ratio of 0.1 to the radioactive clay at a temperature of 60°C for 6 hours after washing with 0.5 M HNO3.

[0065] The present invention will be explained in more detail below through examples, but the following examples are not intended to limit the scope of the invention and should be interpreted as being for the purpose of aiding understanding of the invention.

[0067] Example 1 - Preparation of PAN-based metal ferrocyanide adsorbent

[0068] 170 mL of 0.5 mol / L Ni(NO3)2·6H2O was added to 150 mL of 0.5 mol / L K4[Fe(CN)6]·3H2O and stirred for 2 hours. The mixed solution was then centrifuged to separate the solid and liquid phases, washed several times with DI water, dried at 70 °C for 24 hours, and milled. Subsequently, a few drops of surfactant Tween-80 were added to the mixed solution and stirred at 50 °C for 1 hour. Next, PAN powder was added to the mixed solution to achieve a NiFC / PAN weight ratio of 1, and the mixture was stirred at 50 °C for 5 hours. Finally, spherical beads with a diameter of 2 mm were prepared by dropping the mixed solution into distilled water using a 1 mm tip. The finished beads were washed several times with DI water and dried at 60 °C for use.

[0070] Examples 2 to 4: Preparation of PAN-based metal ferrocyanide adsorbents

[0071] A PAN-based metal ferrocyanide adsorbent was prepared in the same manner as in Example 1 above, but with the NiFC / PAN weight ratios varied to 2, 3, and 4, respectively.

[0073] Experimental Example 1 - Evaluation of Cesium Adsorption Rate and Maximum Adsorption Capacity According to NiFC:PAN Weight Ratio

[0074] Referring to Figure 3, the results of evaluating the adsorption rate of cesium according to the weight ratio of NiFC to PAN (1:1, 2:1, 3:1, 4:1) in distilled water (DI water) and 0.5 M HNO3 can be seen. It was found that the adsorption rate and amount of NiFC-PAN increased significantly from the NiFC:PAN weight ratio (1:1) adsorbent to the weight ratio (2:1) adsorbent. However, since the amount of adsorption is similar at weight ratios above (2:1), the optimal NNiFC:PAN weight ratio (2:1) adsorbent was selected and used.

[0075] Referring to Figure 4, the results of evaluating the maximum adsorption capacity of NiFC powder and NiFC-PAN bead adsorbents in distilled water (DI water) and 0.5 M HNO3 can be seen (the maximum adsorption capacity was calculated based on the amount of NiFC powder contained in PAN, not the total weight). In distilled water (DI water), the maximum adsorption capacities of NiFC powder and NiFC-PAN bead adsorbents were similar, at 178.77 and 173.48 mg / g, respectively. However, in 0.5 M HNO3, which is used as a washing solution, the NiFC powder and NiFC-PAN bead adsorbents showed 121.30 and 173.48 mg / g, respectively, indicating that the NiFC-PAN bead adsorbent exhibited higher adsorption performance. This indicates that the adsorption capacity of the NiFC-PAN adsorbent for cesium is enhanced in 0.5 M HNO3.

[0077] Experimental Example 2 - Evaluation of Cesium Desorption from Clay under Various Temperature Conditions

[0078] Referring to Figure 5, it can be seen that when treated at a high temperature of 60°C, the cesium desorption efficiency increases from 30.3% to 51.7% as the HNO3 concentration increases from 0.1 to 1.0 M. Through this, it can be seen that cesium desorption increases significantly as the HNO3 concentration and treatment temperature increase, and that the mobility of cesium in clay minerals can be improved by increasing the temperature and HNO3 concentration.

[0080] Experimental Example 3 - Evaluation of Cesium Desorption from Clay (HBT) According to the Amount of Adsorbent

[0081] Referring to Figure 6, it can be seen that as the amount of PAN-based metal ferrocyanide adsorbent added increases, the cesium desorption increases, and it can be confirmed that the highest cesium desorption rate of 73.3% is observed with a weight ratio of 10% NiFC-PAN beads.

[0083] Experimental Example 4 - Practical Applicability Experiment on Tetragenic Cesium-Contaminated Clay

[0084] To verify the cesium desorption effect according to the present invention for radioactive soil decontamination, an adsorbent in the form of PAN-based metal ferrocyanide beads corresponding to 10% of the total weight of the contaminated soil in 0.5 M HNO3 was added, and the desorption efficiency was evaluated on clay (HBT) contaminated with the radionuclide cesium-137 under conditions of 60°C, and the radioactivity of the initially contaminated clay was 209 Bq / g, which was less than 0.1 ppb.

[0085] Referring to Table 1 below, it can be seen that the desorption efficiency of cesium through the first step according to one embodiment of the present invention is significantly improved compared to the conventional method. More specifically, when only acid washing was performed without using the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention, the radioactivity decreased from 209 Bq / g to 70 Bq / g and the removal efficiency was 66.5%. However, when the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention was added simultaneously with acid washing, the radioactivity of the contaminated clay decreased from 209 Bq / g to 27 Bq / g and the removal efficiency was significantly improved to 87.1%. Through these results, it can be seen that desorption based on the PAN-based metal ferrocyanide bead-shaped adsorbent according to the present invention can significantly improve decontamination efficiency by preventing the re-adsorption of radioactive cesium in contaminated clay. Furthermore, referring to Table 2 below, it can be seen that the desorption rate of the first step according to the present invention is significantly superior compared to the conventional clay desorption technology using acidic solutions and cations.

[0086] Radioactivity in clay (Bq / g) Desorption efficiency (%) Initial 137Cs-contaminated soil 209 - Acid treated 70 66.5 Acid-treated NiFC-PAN 27 87.1

[0088] Desorption Solution (g / mL) Desorption efficiency Clay (mmol / g synthetic, Bq / g actual) Ref. Acidic solutions 1 M HCI 58% Weathered biotite from Fukushima (370 Bq / mL) Mukai et al., 2016 1 M ammonium acetate (1 / 40) 3% Fukushima soil (130 Bq / g) Hirose et al.,2015 Cations 0.2 M CaCl2 and MgCl2 (1 / 1000) 23 and 37% Montmorillonite (0.013 mmol / g) Fukushi et al.,2014 0.001 M KCl and cation exchange resin 10 ml (1 / 80) 47% Fukushima soil (22.3 Bq / g) Murota et al.,2016 Acidic solution and Adsorbents 0.5 M HNO3and 10 wt.% NiFC-bead 87.10% Cs contaminated soil (209 Bq / g) The present invention

Claims

Claim 1 A method for decontaminating soil contaminated with radioactive nuclides, comprising: a first step of introducing 0.1 to 1.5 M of an inorganic acid and a PAN (Polyacrylonitrile)-based metal ferrocyanide adsorbent into the soil contaminated with radioactive nuclides to desorb the radioactive nuclides from the contaminated soil and adsorb the desorbed radioactive nuclides onto the PAN-based metal ferrocyanide adsorbent; and a second step of separating the PAN-based metal ferrocyanide on which the radioactive nuclides are adsorbed; wherein the first step is characterized by introducing the PAN-based metal ferrocyanide adsorbent into the soil contaminated with radioactive nuclides in a weight ratio of 1:0.001 to 0.2, and the PAN-based metal ferrocyanide adsorbent in the second step is characterized by being spherical in size of 0.5 to 5 mm. Claim 2 A method for decontaminating radioactive nuclide-contaminated soil according to claim 1, characterized in that the radioactive nuclide-contaminated soil comprises clay minerals having high selectivity and strong binding power for radioactive cesium. Claim 3 delete Claim 4 A method for decontaminating radioactive nuclide-contaminated soil according to claim 1, characterized in that the first step is performed at 20 to 80 ℃. Claim 5 A method for decontaminating radioactive nuclide-contaminated soil according to claim 1, wherein the PAN-based metal ferrocyanide adsorbent of the first step is prepared by mixing a metal ferrocyanide selected from the group consisting of Ni, Co, Cu, and Ti, which has excellent adsorption capacity for PAN-based compounds and cesium, in a weight ratio of 1:0.5 to 5. Claim 6 delete Claim 7 A method for decontaminating radioactive nuclide-contaminated soil according to claim 2, characterized in that the PAN-based metal ferrocyanide adsorbent of the first step prevents cesium desorbed by washing with acid from being re-adsorbed onto clay minerals. Claim 8 delete Claim 9 delete Claim 10 delete

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