Chelating agents, such as EDTA, NTA, and citric acid, possess the capacity to establish complexes with radionuclides, potentially enhancing the migration of such radionuclides from the disposal sites. Hence, quantification of these chelating agents in radioactive wastes is required to ensure secure disposal protocols. The determination of chelating agents in radioactive wastes is mainly composed of two steps, e.g. extraction and detection. However, there are little information on the extraction of the chelators in various radioactive wastes. We endeavored to optimize the extraction conditions for citric acid (CA) found within concrete, a prevalent component in the context of dismantled waste materials. Given the inherent high solubility of CA in water, we applied an aliquot of deionized water to the concrete and conducted a one-hour ultrasonic leaching procedure to facilitate chelate extraction. Subsequently, following the preparation of the concrete leachate via vacuum filtration and centrifugation to yield a clarified solution, we quantified the concentration of CA within the solution using Ion Chromatography (IC). To enhance leaching efficiency, we examined the % recovery variation with respect to the pH of the leaching solution. The optimized extraction method will be applied to diverse chelating agents and radioactive waste categories.

Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA) are widely used in industry and agriculture as water softeners, detergents, and metal chelating agents. In wastewater treatment plants, a significant amount of chelating agents can be discharged into natural waters because they are difficult to degrade. Since those compounds affect the mobility of radionuclides or heavy metals in decontamination operations at nuclear facilities and radioactive waste disposal, quantification of the amount of ligand is very important for safe nuclear waste management. To predict the behavior of the main complexation in sample matrices of radioactive wastes, it is essential to evaluate the distribution of the metal-chelating species and their stabilities in order to develop analytical techniques for quantifying chelating agents. We have investigated to collect information on the pH speciation of metal chelation and the stability constants of metal complexes depending on three chelating agents (EDTA, DTPA, and NTA). For example, Zhang’s group recently reported that the initial coordination pH of Cu(II) and EDTA4− is delayed with the addition of Fe(III), and the pH range for the stable existence of [Cu(EDTA)]2− is narrowed compared to when it is alone in the sample matrix. The addition of Fe(III) clearly impacts the chemical states of the Cu(II)-EDTA solution. Additionally, Eivazihollagh’s group demonstrated differences in the speciation and stability of Cu(II) species between Cu(II) and three chelating ligands (EDTA, DTPA, and NTA). This study will be greatly helpful in identifying the sample matrix for binding major chelating agents and metals as well as developing chemically sample pretreatment and separation methods based on the sample matrix. Finally, these advancements will enable reliable quantitative analysis of chelating agents in decommissioning radioactive wastes.

Low- and intermediate-level radioactive waste for permanent disposal often contains organic complexing agents, so-called chelating agents. Organic complexing agents, which are polycarboxylic acids, can increase the mobility of radionuclides into the environment by forming water-soluble complexes with most heavy metals. Therefore, analyzing the complexing agents in radioactive waste is crucial for comprehensive management of nuclear wastes. According to regulatory guidelines, specifically Notice No. 2021-16 issued by the Nuclear Safety and Security Commission, the determination of chelating agent content in radioactive waste materials is required to ensure proper management and safe disposal. However, only a few methods are available to analyze the chelators in various matrices such as concrete, metals, soil, and mixed solid wastes like plastics, vinyl, and rubber. Recently, we found a UV-Vis method based on an enzymatic reaction is inadequate for analyzing citric acid in radioactive waste with a complex matrix like concrete. To address this, we developed a method to determine the contents of EDTA and NTA using a UV-Vis spectrophotometer and citric acid using ion chromatography. The results showed good validity and reliability to determine the chelating agents in various radioactive wastes.

Low- and intermediate-level radioactive wastes have been disposed of in the first-phase deep underground silo disposal at Gyeongju in South Korea. These radioactive wastes contain harmful radionuclides such as Uranium-238 (238U), which can pose long-term and deleterious effects on humans and the natural environment. Ethylenediaminetetraacetic acid and isosaccharinic acid, which can be formed via cellulosic waste degradation under high alkaline conditions might considerably enhance the transport behavior of 238U with the intrusion of rainwater and groundwater. In this study, the engineered barriers (concrete and grout) and natural barriers (sedimentary rock and granite) were used to investigate the 238U transport behavior in artificial cementitious porewater of State I (pH 13.3) and State II (pH 12.5) based on groundwater or rainwater. The surface properties and geochemical compositions of barrier samples were characterized using XRD, XRF, SEM-EDX, and BET. The transport behaviors of 238U in various solution conditions were observed by sorption distribution coefficient (Kd) at a range of initial chelating agents concentration (10-5-10-2 M). The sorption behavior of 238U was retarded more in the engineered rock barriers than in the natural rock barriers. The mobility enhancement of 238U was more significant in State I than in State II. In comparison with the absence of chelating agents, negligible changes in the Kd values of 238U were observed at less than initial chelating agent concentrations of 10-4 M. However, the Kd values of 238U were significantly reduced at initial chelating agent concentrations higher than 10-3 M. Therefore, these experimental findings show that the transport behavior of 238U into the geo- and bio-sphere could be accelerated by the presence of chelating agents and the type of cement degradation states.

Chelating agents, such as EDTA, NTA, and citric acid, can form radionuclide-chelate complexes that may enhance the migration of radionuclides from disposal sites. Therefore, the contents of these chelators in radioactive wastes are determined for the secured disposal. In this study, a rapid automated method using ion chromatography (IC) is described for analysis of chelating agents. The method enables direct use of a sample solution for the measurement unlike colorimetric or enzymatic methods. In these cases, lots of chemicals are consumed to convert the chelating agents to be UV-Vis-active compounds during the pretreatment process and finally, absorbance or transmittance are measured at specific wavelength using UV-Vis spectrophotometer. In particular, the enzymatic method for determination of citric acid, because of its strong dependence on experimental conditions including sample types and chemical reagents may produce inconsistent results. The automated system using IC allows a laboratory to directly measure the amount of citric acid while reducing total analysis time and increasing efficiency. In addition, this method is capable of detecting a trace amount of citric acid, thereby lowering LOD and LOQ values.

Phytoextraction은 식물을 이용하여 환경 정화하는 기술로서 금속으로 오염된 토양을 정화하는 것이다. 토양에 존재하는 금속의 추출을 용이 하기 위해서 현재 다양한 킬레이트가 사용되고 있다. Phytoextraction이 경제적이고 친환경적인 장점이 있지만 고농도로 오염된 지역에서는 적용이 어려운데 이는 식물이 이러한 지역에서 살아남기 어렵기 때문이며 이러한 문제점을 해결하는 것이 본 연구의 목적이다. 연구 대상의 금속으로서 수은을 선택하였고, 킬레이트는 아미노산인 시스테인과 히스티딘, 작은 크기의 유기산으로서 citric acid, malic acid, succinic acid, oxalic acid, 그리고 ethylenediamine (EDA)를 선택하였으며, EDTA는 비교 대상으로 본 연구에 사용되었다. 다양한 농도의 수은을 포함하는 배지에 식물을 키우면서 여러 킬레이트가 식물의 뿌리 성장에 미치는 영향을 분석하였다. 수은에 의한 식물의 성장 억제는 시스테인과 EDA에 의해서 완화되었으며 히스티딘과 citrate는 별 다른 영향이 없었다. Malate, succinate, 그리고 oxalate는 수은에 의한 식물 성장 억제를 더 촉진시켰다. 따라서 수은의 식물성장억제를 완화시켜주는 시스테인과 EDA는 고농도의 수은으로 오염된 지역에서 식물의 성장이 가능하도록 유용하게 사용될 수 있을 것이다.