Selective Cesium Removal from Water over a Wide pH Range using Potassium Metal Thiostannates = 새로운 칼륨 금속 티오스타네이트를 사용한 광범위한 pH 범위에서의 선택적 세슘 제거

다국어 초록 (Multilingual Abstract)

In order to meet the increasing energy demand and alleviate global warming, nuclear power, as a relative clean energy, has been receiving attention again. However, the water consumption of nuclear power plants (NPPs) is much greater than conventional coal-fired power plants with the same capacity. Of greater concern is the resulting wastewater, with pH values ranging from extremely acidic to strongly alkaline, which contains a multitude of radionuclides. Among them, 137Cs, as the major fission product, poses a threat to human health due to its production of gamma rays and high-energy beta particles and a long half-life. Furthermore, the coexistence of highly concentrated nonradioactive ions in wastewater makes 137Cs separation extremely challenging. Unlike conventional adsorbents, metal sulfides have an inherently high affinity for Lewis soft acid Cs+ because of the presence of Lewis soft base –S in their framework. However, their adsorption performance is significantly reduced under acidic and alkaline conditions, and the reason has not been revealed yet. In this thesis, we addressed the design of novel metal sulfide adsorbents with high adsorption capacity and high selectivity toward Cs over a wide pH range and systematically investigated the adsorption mechanism through multiple characterizations.
First, potassium antimony thiostannate (KSbSnS-2) is synthesized by doping Sb3+ with lone pair electrons into the Sn-S matrix. Compared with Sn4+, Sb3+ has a relatively lower charge valence and a larger ionic radius, thereby expecting a more Lewis basic Sb3+-containing thiostannate framework that can improve the adsorption capacity for Cs+. Furthermore, the addition of Lewis basic Sb3+ with lone pair electrons could allow the metal sulfide structure to withstand a more basic environment and provide additional adsorption sites. As a result, KSbSnS-2 showed a high adsorption capacity (358 mg/g) and distribution coefficient (1.59 × 105 mL/g) toward Cs+. The adsorption performance was good over a wide active pH range (1–12), even in extreme alkaline conditions in particular (Kd = 3.26 × 104 mL/g at pH 12). However, the distribution coefficient decreased significantly under acidic conditions.
To address this issue, a novel potassium aluminum thiostannate (KAlSnS-3) adsorbent was designed with the hypothesis that the release of structural Al3+ would contribute to the Cs+ adsorption. Under highly acidic and alkaline conditions, the release of Al3+ due to the low stability of Al-S bonds compensates for a decrease in adsorption capability, a general tendency in harsh conditions. In addition, the lower valence of Al3+ than that of Sn4+ results in a negative charge, which is then balanced by extra K+ ions. The K+ ions intercalated between 2D layers act as the primary active species for Cs+ adsorption. As a result, KAlSnS-3 demonstrated excellent adsorption performance across a broad pH range (1–13) (Kd: 1.04 × 104 mL/g at pH 2, 2.97 × 104 mL/g at pH 12), and high selectivity for Cs+. However, due to the large size difference between Al3+ and Cs+, fewer Cs+ were adsorbed than H+ under acidic conditions.
To further improve the adsorption performance for Cs+ under acidic conditions, a novel SnS2-type potassium calcium thiostannate (KCaSnS) was developed by introducing Ca2+ with a large radius (1.00 Å) and low valence into the Sn-S matrix. The release of structural Ca2+ in addition to the interlayered K+ under acidic conditions could enhance the adsorption of Cs+ because of their similar ionic radius. As a result, KCaSnS showed high Cs+ adsorption capacity at acidic (qm: 235 mg/g) and neutral (qm: 273 mg/g) solutions. KCaSnS also exhibited high selectivity for Cs+ even in acidic artificial seawater (Kd = 2.95 × 103 mL/g).
Finally, the mechanism behind the excellent adsorption performance of KSbSnS-2, KAlSnS-3, and KCaSnS was discussed, respectively, on the basis of the concepts of electrostatic interactions and chemical affinity, supported by the evidence of crystallinity, elemental concentration, and binding energy of electrons.

목차 (Table of Contents)

• Chapter 1. General Background 1
• 1.1. Background 1
• 1.2. Characteristics of radioactive wastewater 2
• 1.2.1. Classification of radioactive wastewater and discharge limits 2
• 1.2.2. Wide pH range of radioactive wastewater 3
• 1.2.3. High concentration of coexisting nonradioactive ions 4
• 1.3. Characteristics and sources of radioactive cesium 5
• 1.4. State-of-the-art decontamination technology for radioactive wastewater 8
• 1.5. Popular adsorbents for Cs+ removal 10
• 1.6. Research objectives 12
• Chapter 2. Will the Doping of Lewis Basic Sb3+ Enhance Cs+ Adsorption Performance 14
• 2.1. Introduction 14
• 2.2. Materials and method 15
• 2.2.1. Chemicals and materials 15
• 2.2.2. KSbSnSs synthesis 16
• 2.2.3. Characterization 16
• 2.2.4. Adsorption 17
• 2.3. Results and discussion 19
• 2.3.1. Sb dosage effect 19
• 2.3.2. KSbSnS-2 characterization 23
• 2.3.3. Adsorption kinetics 28
• 2.3.4. Adsorption isotherms 30
• 2.3.5. Effect of pH 35
• 2.3.6. Competing ions effect 38
• 2.3.7. Cs adsorption mechanism on KSbSnS-2 42
• 2.4. Conclusions 48
• Chapter 3. Can Dopant Al3+ Act as an Ion-exchanging Cation in Addition to K+ at Acidic and Alkaline Conditions 49
• 3.1. Introduction 49
• 3.2. Material and methods 50
• 3.2.1. Chemicals and materials 50
• 3.2.2. Hydrothermal synthesis of KAlSnS-3 50
• 3.2.3. Characterization techniques 51
• 3.2.4. Adsorption experiments 51
• 3.3. Results 53
• 3.3.1. General characterization of KAlSnS-3 53
• 3.3.2. Kinetics study 58
• 3.3.3. Adsorption isotherms 61
• 3.3.4. pH-dependent Cs+ adsorption 65
• 3.3.5. Effect of competing ions 67
• 3.4. Discussion 71
• 3.4.1. Driving forces for adsorption 71
• 3.4.2. Mechanism of Cs+ adsorption at different pH 72
• 3.4.3. Effect of Al3+ release on the reusability of KAlSnS-3 83
• 3.5. Conclusions 85
• Chapter 4. Will the Elution of Large-size Dopant Ca2+ Enhance Cs+ Adsorption under Acidic Conditions 86
• 4.1. Introduction 86
• 4.2. Material and methods 87
• 4.2.1. Chemicals and materials 87
• 4.2.2. Hydrothermal synthesis of KCaSnS 87
• 4.2.3. Characterization techniques 88
• 4.2.4. Batch adsorption experiments 88
• 4.3. Results 90
• 4.3.1. Characterization of KCaSnS 90
• 4.3.2. Kinetics study of Cs+ adsorption 94
• 4.3.3. Adsorption isotherm study 97
• 4.3.4. pH-dependent adsorption 101
• 4.3.5. Selectivity study 104
• 4.4. Discussion 108
• 4.4.1. Adsorption mechanism 108
• 4.4.2. Effect of the release of Ca2+ on Cs+ adsorption 115
• 4.4.3. Reusability test 119
• 4.5. Conclusions 122
• Chapter 5. Conclusions and Outlook 123
• 5.1. Conclusions 123
• 5.2. Prospects for future research 125
• References 127
• List of Publications 139
• Acknowledgments 143