Immobilization of selenium and iodine asradionuclide surrogates in cement relatedmaterials for burying after solidification

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九州大学学術情報リポジトリ

Kyushu University Institutional Repository

Immobilization of selenium and iodine as radionuclide surrogates in cement related materials for burying after solidification

郭, 柄霖

http://hdl.handle.net/2324/1959101

出版情報：九州大学, 2018, 博士（工学）, 課程博士バージョン：

権利関係：

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Immobilization of selenium and iodine as radionuclide surrogates in cement related materials for burying after

solidification

By Binglin GUO

A thesis submitted to Kyushu University

for the degree of Ph.D

September 2018

Department of Earth Resources Engineering Graduate School of Engineering

Kyushu University

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Abstract

The confinement of radionuclides or hazardous elements is an important issue for preventing environmental contamination and helping to maintain safe ecosystems for living organisms. Among the radioactive isotopes, the ¹²⁹I and ⁷⁹Se isotope are considered as remarkable environmental risk related to nuclear waste storage or disposal because of their relatively long half-life (1.57×10⁷ years and 2.95×10⁵, respectively) and high mobility in soil and aqueous system. Since radioactive isotopes are a relatively emergent contaminant and nuclear wastes are usually immobilized in cement, immobilization of I and Se speciesto cementitious materials is of great interest. As a major and active component in hydrated cement, ettringite is assumed to play a role in immobilization of toxic anions because of its ion-exchange ability. This thesis investigated the immobilization mechanisms and properties of iodine and selenium as radionuclide surrogatesin ettringite. In addition, the stability of ettringite after uptake toxic anions is also discussed. After accumulation of radionuclides from a contaminated source, ettringite should be stabilized before landfilling for long time storage.

An investigation into developing the alternative method to stabilize hazardous wastes by using industrial byproduct/wastes to synthesis glass-ceramics is also conducted.

In Chapter 1, the background information of selenium and iodine, including the history, property, toxicity of these elements, as well as their radionuclide isotopes property was elucidated. The characteristic of ettringite and its related minerals are also presented. Based on the presented background, the purpose and motivations of this thesis were discussed.

Although ettringite is a crucial material in terms of Se immobilization, the immobilization mechanisms, atomic configuration, and intercolumn structure of Se sorbed in ettringite are unclear. In Chapter 2, the immobilization mechanism of Se oxoanions was evaluated through structural insight into ettringite. It is contrasting between SeO32– and SeO42– in chemical property of the solid residues after immobilization. The oxoanion exchange with

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structural SO42– is the main mechanism for immobilization of SeO42–. In contrast, SeO32– is easily immobilized to form inner-sphere complexes in ettringite. In addition, it is necessary to reveal the SeO32– complexation sites for understanding the mechanisms in immobilization of SeO32–. Based on the characterization results with the bond valence theory, the location sites of sorbed SeO32– in ettringite structure were proposed.

In Chapter 3，the effect of SO42− on SeO42− immobilization was investigated, because SO42− is one of the major ions in aqueous and soil environments and it is easily incorporated in ettringite. The formation mechanisms of SeO42−

-substituted ettringite with and without SO42−

were systemically elucidated. When SO42−

and SeO42−

coexisted, both oxoanions were immediately coprecipitated with ettringite after adding a Ca source. Although SO42− was partially substituted by SeO42− in ettringite, no other phases were formed during the process.

Without SO42−

, SeO42−

- substituted hydrocalumite was formed as an intermediate, confirmed by X-ray diffraction peak at d 2.878 Å, as well as by scanning electron microscopy with energy dispersive X-ray spectroscopy and zeta potential measurements. It is clear that the incorporation mechanism of SeO42− in ettringite is dependent on coexisting SO42− in aqueous environments.

Understanding of immobilization mechanisms of I^– and IO3– in ettringite is particularly important in hazardous and radioactive waste management. In Chapter 4, the immobilization performance of I^– and IO3–

in ettringite is discussed. Based on the water chemistry, XRD, FTIR, and EXAFS results, I^– is difficult to be incorporated in ettringite structure. Meanwhile IO3– exhibits high affinity to be incorporated in ettringite through hydrogen bond and electrostatic force via anion exchange with SO42–. In order to figure out the immobilization mechanism of I^– and IO3–

via ettringite, various anions with different ionic radius and valence were compared to co-precipitate with ettringite. According to the different anions' properties and the structure of ettringite, the immobilization mechanism of various anions via ettringite is finally proved to relate to the subjected anions' hydration ability and ionic radius. The

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anions which are more favorably hydrated with smaller ionic radius than sulfate could be easily incorporated in ettringite.

In Chapter 5, the chemicalstability of ettringite, which has uptaken toxic anions, were evaluate under the aqueous solution. After coprecipitation, selenate ettringite was formed.

However, when this mineral was soaked in a sulfate solution, the destructuralization was observed. In addition, the columnar structure of the destructuralized ettringite was maintained as confirmed via its characterization by a variety of techniques. The destructuralization will cause the immobilized toxic anions re-diffused. The pure ettringite is not ideal as the final product to accommodate hazardous ions.

In general, to reduce the volume of bulky sludge after co-precipitation, heat treatment is applied. In Chapter 6, the thermal stability of SeO42– bearing ettringite was investigated.

SeO42– bearing ettringite was gradually transformed to a new crystal phase until 600 °C, which should be assigned to 6CaO·Al2O3·3SeO2. Based on the results of extended X-ray adsorption fine structure (EXAFS) and ²⁷Al nuclear magnetic resonance (²⁷Al-NMR), the partial structure of this new phase was proposed. That is, the SeO42− gradually reduced to SeO32−

under high-temperature calcination and the coordination environment of Se oxoanions also changed from outer-sphere to inner-sphere when the SeO42– was reduced to SeO32–. This phenomenon is caused by the dehydroxylation, providing electron donor sites on the ettringite.

Furthermore, calcination of SeO42– bearing ettringite at 800 °C much improved the stability of selenium. This is due to inner-sphere complexation, which is more resistant to remobilization than outer-sphere complexes.

Development of novel and efficient method to stabilize spent absorbents that contain highly toxic ions is urgently needed. In the Chapter 7, the use of industrial byproducts as raw materials to produce ceramics to treat toxic waste was investigated. As a typical oxoanion absorbent, SeO42−-doped ettringite was mixed with granulated blast furnace slag (GBFS) and silica fume (SF) and then calcined at various temperatures to produce glass-ceramics. Above

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800 °C, the amorphous mixture was converted to glass-ceramics, which were subjected to the toxicity characteristic leaching procedure (TCLP) test. The synthesized ceramics exhibited that lower than 0.1 mg/L of selenate was leached out, and no selenium was evaporated during calcination. The X-ray photoelectron spectroscopy (XPS) revealed the stabilization mechanism of selenium is based on encapsulation by ceramicrete. In addition, the ceramic materials exhibited excellent chemical stability at pH 2~12. These results showed that industrial byproducts can be successfully applied to produce ceramics for immobilization and storage of hazardous wastes.

Finally, in Chapter 8, the main conclusions of this thesis were summarized to give insights on immobilization of I and Se species in ettringite. However, the immobilization efficiency is depending on the hydration ability of subjected anions. Furthermore, the stability of ettringite is not ideal under the acidic and neutral pH, although this mineral exhibits high efficiency to incorporate the anionic species of I and Se. Thus, development of physical or chemical encapsulation of ettringite in economic matrix might be an alternative way to solve this problem.

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VI Contents

Cover ... I Abstract ... II Contents... VI List of tables ... XII List of figures ... XIII Chapter 1

Introduction ... 1

1.1. The chemistry of selenium and radioactive selenium ... 2

1.1.1 The history and nature distribution of selenium ... 2

1.1.2. Selenium and its radioactive isotopes ... 2

1.1.3. The chemistry of selenium ... 4

1.2. The chemistry of iodine and radioactive iodine ... 6

1.2.1. The history and importance of iodine... 6

1.2.2. Iodine and its radioactive isotopes ... 6

1.2.3. The chemistry of iodine ... 8

1.3. Radioactive wastes treatment ... 10

1.3.1. Radioactive wastes classification ... 10

1.3.2. Radioactive wastes immobilization ... 11

1.3.3 Immobilization materials ... 12

1.4. Ettringite ... 14

1.4.1. Nature occurrence and chemical composition ... 14

1.4.2. Crystal structure ... 15

1.5. The objectives and outline of this thesis ... 17

References ... 20

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VII Chapter 2

Selenite and selenate uptaken in ettringite: Immobilization mechanisms, coordination

chemistry, and insights from structure ... 29

2.1. Introduction ... 30

2.2. Experimental ... 31

2.2.1. Sample preparation ... 31

2.2.2. Characterization of solid residues ... 32

2.2.3. EXAFS analysis ... 33

2.2.4 Bond valence analysis ... 34

2.3. Results ... 35

2.3.1 Composition of the SeO32– and SeO42– doped ettringite ... 35

2.3.2 XRD patterns ... 36

2.3.3. FTIR analysis ... 39

2.3.4. TG analysis ... 40

2.3.5. EXAFS analysis of Se k-edge of Se(IV) and Se(VI) doped ettringite ... 43

2.3.6. EXAFS analysis of Ca k-edge of Se(IV)and Se(VI)doped ettringite ... 46

2.4. Discussion ... 47

2.4.1 SeO32– and SeO42– incorporation by ettringite ... 47

2.4.2. Coordination chemistry analysis ... 52

2.5. Conclusions ... 55

References ... 56

Chapter 3 Characterization of the intermediate in formation of selenate-substituted ettringite ... 59

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3. 1. Introduction ... 60

3. 2. Experimental ... 61

3.2.1. Materials ... 61

3.2.2. Incorporation of SeO42− by coprecipitation with ettringite ... 61

3.2.3.1. Powder X-ray diffraction ... 62

3.2.3.2. SEM-EDS ... 63

3.2.3.3. Zeta potential measurement ... 63

3.3. Results ... 63

3.3.1. Selenate immobilization ... 63

3.3.2. Characterization of the solid residues ... 64

3.3.2.1. X-ray diffraction ... 64

3.3.2.2. SEM-EDX observation ... 66

3.3.2.3. Zeta potential measurements ... 69

3.4. Discussion ... 72

References ... 81

Chapter 4 Confinement of iodite and iodate in ettringite: the inspiration of plausible mechanism for various inorganic anions selective incorporation ... 83

4.2. Materials and Methods ... 85

4.2.3. EXAFS analysis ... 86

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4.3. Results and discussion ... 87

4.3.1. Sorption mechanism of iodide/iodate in ettringite ... 87

4.3.2. Selectivity of inorganic anions ... 96

References ... 105

Chapter 5 Structural stability evaluation for selenate bearing ettringite under sulfate solution ... 108

5.2. Materials and methods ... 110

5.2.1. Material preparation ... 110

5.2.2. Material destructuralization and exfoliation ... 111

5.2.3. Characterization ... 111

5.2.3.2. Raman spectroscopy ... 112

5.2.3.3. Scanning electron microscopy/transmission electron microscopy ... 112

5.2.3.4 EXAFS analysis ... 112

References ... 125

Chapter 6 Reduction and phase-transformation of selenate bearing ettringite by calcination for stabilization of ⁷⁹Se oxoanions ... 127

6. 1. Introduction ... 128

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6. 2. Experimental ... 131

6.2.3 EXAFS analysis ... 133

6.2.4. TCLP test ... 134

6.3.1 Synthesized products characterization ... 135

6.3.2 Calcination behavior of selenate-based ettringite ... 140

6.3.3 Fine structure analysis and reduction mechanism ... 144

6.3.4 TCLP leaching results ... 154

References ... 156

Chapter 7 Solidification of ettringite after uptaking selenate as a surrogate of radionuclide in glass-ceramics by using industrial byproducts ... 161

7.2. Experimental ... 164

7.2.1. Selenate-substituted ettringite synthesis ... 164

7.2.2 .Ceramics production ... 164

7.2.3. Solids characterizations... 165

7.2.3.2. SEM observation ... 166

7.2.3.3. TG-DTA analysis ... 166

7.2.3.4. X-ray photoelectron spectroscopy analysis ... 166

7.2.4. TCLP test ... 167

7.3. Results and Discussion ... 167

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7.3.1 Ceramics synthesis and their properties ... 167

7.3.2 Immobilization mechanisms ... 177

7.3.3 Chemical stability ... 182

References ... 185

Chapter 8 Conclusions ... 189

Acknowledgements ... 196

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XII

List of tables

Table 1.1 Selenium naturally occurring isotopes data [8].

Table 1.2 Data of selenium radioactive isotopes [8, 9].

Table 1.3 Data of selenium radioactive isotopes [8, 9, 35].

Table 1.4 Ettringite and its related ettringite group minerals [58, 68-72].

Table 2.1 Elemental compositions in oxyanion-doped ettringite as a function of initial SeO32–

and SeO42– concentrations.

Table 2.2 Se K-edge EXAFS fitting results of SeO32– and SeO42– doped ettringite and standards. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ²), and residual factor (Rf).

Table 2.3 Ca K-edge EXAFS fitting results of SeO32– and SeO42– doped ettringite and standards. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ²), and residual factor (Rf).

Table 4.1 Compositions of Iodine-doped ettringite as a function of initial I^– and IO3–

concentrations.

Table 5.1 Concentrations of Ca, Al, and Se after the soaking experiment as a function of the initial SO42– concentrations with the Ca: Al: Se molar ratio in the solid residues.

Table 5.2 Ca K-edge EXAFS fitting results of pure sulfate ettringite, selenate ettringite and the soaked compounds in 10 mM Na2SO4. Coordination number (CN), interatomic distance (R), and Debye-Waller factor (ơ²).

Table 6.1 Se K-edge EXAFS fitting results of uncalcined, 200 °C calcined, and (c) 800 °C calcined selenate bearing ettringite. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ²), and residual factor (Rf).

Table 7.1 Chemical composition (wt%)and specific density, d (g/cm³) of granular blast furnace slag (GBFS) and silica fume (SF) determined by XRF.

Table 7.2 Surface molar ratios of uncalcined and calcined products at 200 and 800 °C estimated by XPS results.

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List of figures

Fig. 1.1 Eh–pH stability diagram of Se at 25 °C, 1.013 bars and activity of Se = 10 mg/L [17].

Fig. 1.2 Eh–pH stability diagram of I at 25 °C, 1.013 bars and activity of I = 10 mg/L [17].

Fig. 1.3 The process for radioactive waste managing.

Fig. 1.4 Structure of ettringite. (a) Perpendicular to c-axis shown columns and channels, (b) projection of ettringite structure, (c) expended the dash line regions in (a) and (b) (Moore et al., 1970, Taylor, 1973).

Fig. 2.1 XRD patterns of solid residues after coprecipitation of (a) SeO32– and (b) SeO42– with ettringite at various initial concentrations. Numbers in brackets indicate the equilibrated Se concentrations.

Fig. 2.2 Changes of lattice parameters (a) a and (b) c of ettringite reacted with SeO32– and SeO42– at various initial concentrations. Numbers in brackets indicate the equilibrated Se concentrations and whole water molecules.

Fig. 2.3 FTIR vibration spectra of ettringite reacted with (a) SeO32– and (b) SeO42– at various initial concentrations.

Fig. 2.4 TG curves of ettringite reacted with (a) SeO32– and (b) SeO42– at various initial concentrations.

Fig. 2.5 Se K-edge XANES spectra of SeO32– and SeO42–-doped ettringite with reference compounds CaSeO3 and CaSeO4.

Fig. 2.6 (a) k³-weighted Se K-edge EXAFS data of SeO32– and SeO42– doped ettringite and reference compounds. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

Fig. 2.7 (a) k³-weighted Ca K-edge EXAFS data of SeO32– and SeO42– doped ettringite and a reference compound. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

Fig. 2.8 FTIR vibration spectra of ettringite and the related compounds.

Fig. 2.9 Curve fitted FTIR spectra of the 2750~4000 cm^-1 region for (a) pure sulfate ettringite; (b) 15% selenite-doped ettringite; (c) pure selenate ettringite.

Fig. 2.10 Schematic illustration of coordination of SeO32– in ettringite. Numbers indicate the sum of bond valence values of different atoms.

Fig. 3.1 Changes in (a) Se, (b) Al, (c) Ca, (d) S, and (e) pH with increasing time during immobilization of SeO42– with and without containing SO42– in solutions.

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Fig. 3.2 Changes in X-ray diffraction patterns for solid residues during co-precipitation of SeO42– with ettringite in the presence of SO42– in solution.

Fig. 3.3 Changes in X-ray diffraction patterns for solid residues during precipitation of SeO42–

with ettringite in the absence of SO42– in solution. The region in (a) are expanded in (b).

Fig. 3.4 SEM images for the solid residues after (a) 2 min, (b) 5 min, (c) 15 min, (d) 30 min, (e) 45 min, (f) 60 min, and (d) 120 min precipitation of SeO42– without SO42–.

Fig. 3.5 SEM-EDX results for the selected square in the solid residues after (a) 2 and (b) 120 min. The result of digestion experiment of solid residue was shown in (b) for comparation.

The Al was normalized to 2.

Fig. 3.6 Time course of zeta potentials for the solid residues after co-precipitation of SeO42–

with and without SO42– in solution. Numbers are pH values of the solution where the solid residues were suspended.

Fig. 3.7 Zeta potentials for SeO42–-substituted ettringite and Ca(OH)2 against pH.

Fig. 3.8 Changes in the molar ratio (Se+S)/Al in solid phase with increasing time during co-precipitation in the presence and absence of SO42–.

Fig. 3.9 Schematic illustrations for the formation mechanism of AFt–SeO4.

Fig. 4.1. Sorption isotherms of (a) Iodate and (b) Iodide on ettringite after equilibration. Lines represent linear regression of the data points.

Fig. 4.2 XRD patterns of solid residues after coprecipitation of (a) I^– and (b) IO3–

with ettringite at various initial concentrations. Changes of lattice parameters (c) a and (d) c of I^– and IO3– doped ettringite.

Fig. 4.3 k³-weighted I L3-edge EXAFS Fourier transforms of IO3– doped ettringite and a reference compound (not corrected for phase shift) with both raw (solid lines) and fitted data (dash lines). Schematic illustration of I coordination environment is also exhibited.

Fig. 4.4 FTIR vibration spectra of (a) ettringite reacted with various anionic species with 15 mM (I^– and IO3–

) and 1 mM (B(OH)4–

, ClO4–

, CrO42–, SeO42–, WO42–, and AsO43–), (b) curve fitted FTIR spectra of the 2800~4000 cm^-1 region for each compounds.

Fig. 4.5 XRD patterns of solid residues after coprecipitation of 1 mM ClO4–

, CrO42–, WO42–, SeO42– B(OH)4–, and AsO43–.

Fig. 4.6 The immobilization efficiency of (a) various 1mM anions coprecipitation with ettringite, Correlationship between immobilization efficiency and (b) ionic radius, (c) Jones–Dole coefficient B coefficient, and (d) geometrical factor (∆GHB).

Fig. 5.1 XRD patterns of the solid residues after soaking pure selenate ettringite in 0-10 mM sulfate. Inset: perpendicular to c-axis shows the columns and channels of selenate ettringite

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XV with the (100) and (110) plane.

Fig. 5.2 SEM images of the solid residues (a) before soaking and (b) after soaking.TEM images of the solid residues (c) before and (d) after soaking.

Fig. 5.3 Zeta potentials of pure selenate ettringite after soaking at various initial SO42–concentrations.

Fig. 5.4 Raman spectra of soaked ettringite and the related compounds.

Fig. 5.5 (a) k³-weighted Ca K-edge EXAFS data of solid residues before soaking and after soaking and reference compounds.(b) The corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

Fig. 5.6 Schematic illustration of the selenate ettringite synthesis and destructuralization in sulfate solution.

Fig. 6.1 XRD patterns of selenate ettringite and the calcined products at 200-800 ^oC.

Fig. 6.2 FTIR vibration spectra of calcined ettringite and the related compounds. The region in (a) are expanded in (b).

Fig. 6.3 SEM images of (a) pure selenate ettringite and samples calcined at (b) 200 °C (c) 400 °C. (d) 600 °C (e) 800 °C, and (f) 900 °C.

Fig. 6.4 Thermogravimetric analysis of pure selenate ettringite.

Fig. 6.5 (a), (b) and (c) TEM image of calcined ettringite at 800 ôC; (d) HAADF STEM image of calcined ettringite at 800 ôC; (e-h) EDS mapping of O, Ca, Al, and Se over the area in (d) of calcined ettringite at 800 ôC.

Fig. 6.6 Se K-edge XANES spectra of calcinated selenate ettringite under various temperatures and the related standards (NaSeO3 and NaSeO4).

Fig. 6.7 (a) k³-weighted Se K-edge EXAFS spectra of SeO32– and SeO42– doped ettringite and reference compounds. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

Fig. 6.8 (a) k³-weighted Ca K-edge EXAFS spectra of SeO32– and SeO42– doped ettringite and a reference compound. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).

Fig. 6.9 ²⁷Al NMR spectra for the calcined solid residues of selenate bearing ettringite at (a) 200 °C, (b) 600 °C calcined, and (c) 800 °C.

Fig. 6.10 Schematic illustrations for the reductive phase transformation mechanism of selenate bearing ettringite via calcination at higher than 800 °C.

Fig. 6.11 Total and leached amounts of Se from uncalcined and calcined selenate-bearing ettringite depending on the calcination temperatures.

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Fig. 7.1 XRD patterns of raw materials used for the production of ceramics. Numbers indicate the miller index and 2θ of the main diffraction peaks of the confirmed phases.

Fig. 7.2 SEM images of (a) silica fume, and (c) GBFS. The squares in (a) and (c) are expanded in (b) and (d).

Fig. 7.3 Thermogravimetric analysis of (a) pure ettringite and (b) pure selenate ettringite.

Fig. 7.4 Photographs of calcined products at various temperatures.

Fig. 7.5 (a) XRD patterns of calcined products at various temperatures. Numbers indicate the miller index and 2θ of the main diffraction peaks of the confirmed phases. (b) Rietveld refinement of calcined products at 800 °C.

Fig. 7.6 SEM images of mixtures calcined at (a) 200 °C and (c) 800 °C. The squares in (a) and (c) are expanded in (b) and (d).

Fig. 7.7 Thermogravimetric analysis of the GBFS and SF mixture without selenate ettringite.

Fig. 7.8 Total and leached amounts of Se from heated ceramics depending on the calcination temperatures.

Fig. 7.9 XPS spectra of (a) wide scan, (b) Se 3d, (c)Al 2p, (d)Si 2p, and (e) Ca 2p for uncalcined and calcined products at 200 and 800 °C. Vertical bars indicate (a) 5000 cps, (b) 50 cps, (c)100 cps, (d) 200 cps, and (e) 100 cps.

Fig. 7.10 Leached concentrations of Se from GBFS ceramics calcined at 800 °C at various pH values. The numbers in the figure indicate the mass loss in percentage after leaching.

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Chapter 1

1

Chapter 1 Introduction

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Chapter 1

2

1.1. The chemistry of selenium and radioactive selenium 1.1.1 The history and nature distribution of selenium

Selenium (Se) is discovered in 1818 by a Sweden chemist Jöns Jacob Berzelius and its name derives from the Greek word of "moon" [1]. This element is a metalloid located in the fourth period of the periodic table in the chalcogen group [1, 2]. In nature, selenium is less abundant but ubiquitous in the earth crust [3]. Because it exhibits similar property to sulfur, Selenium is often associated with sulfide minerals through substitution with sulfur such as clausthalite, naumannite, tiemannite, and metallic selenides [4, 5]. Selenium with sulfur minerals has potential to be emitted from volcanoes. Therefore, soils near volcanoes exhibit higher concentrations of selenium. However, selenium is not known to occur in concentrated deposits [3].

1.1.2. Selenium and its radioactive isotopes

Six naturally occurred stable selenium isotopes are confirmed. ⁸⁰Se and ⁷⁸Se which are most abundant selenium isotope account for approximately 73% of all selenium on the Earth. The remained 27% of stable selenium is accounted for ⁷⁴Se, ⁷⁶Se, ⁷⁷Se, and ⁸²Se together (Table 1.1) [6-8].

Audiet al. (2003) has found twenty-four unstable selenium isotopes with half-lives ranging from approximately 20 ms to 295,000 years (Table 1.2) [9].

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Chapter 1

3

Table 1.1 Selenium naturally occurring isotopes data [8].

Isotope Atomic mass (Da) Isotope composition (%)

74Se 73.9224746 0.89(4)

76Se 75.9192120 9.37(29)

77Se 76.9199125 7.63(16)

78Se 77.9173076 23.77(28)

80Se 79.9165196 49.61(41)

82Se 81.9166978 8.73(22)

Table 1.2 Data of selenium radioactive isotopes [8, 9].

Isotope Atomatic mass Half life Decay mode Daughter isotopes

65Se 64.96466 <50 ms β⁺ ⁶⁵As

66Se 65.95521 33 ms β⁺ ⁶⁶As

67Se 66.95009 133 ms β⁺ ⁶⁷As

68Se 67.94180 35.5 s β⁺ ⁶⁸As

69Se 68.93956 27.4 s β⁺ ⁶⁹As

70Se 69.93339 41.1 min β⁺ ⁷⁰As

71Se 70.93224 4.74 min β⁺ ⁷¹As

72Se 71.92711 8.40 days EC ⁷²As

73Se 72.92676 7.15 h β⁺ ⁷³As

75Se 74.92252 119.779 days EC ⁷⁵As

79Se 78.91850 295 000 years β⁺ ⁷⁹As

81Se 80.91799 18.45 min β^– ⁸¹Br

83Se 82.91912 22.3 min β^– ⁸³Br

84Se 83.91845 3.1 min β^– ⁸⁴Br

85Se 84.92225 31.7 s β^– ⁸⁵Br

86Se 85.92427 15.3 s β^– ⁸⁶Br

87Se 86.92852 5.5 s β^– ⁸⁷Br

88Se 87.93142 1.53 s β^– ⁸⁸Br

89Se 88.93645 410 ms β^– ⁸⁹Br

90Se 89.93996 300 ms ND –

91Se 90.94596 270 ms β^– ⁹¹Br

92Se 91.94992 100 ms ND –

93Se 92.95629 50 ms ND –

94Se 93.96049 20 ms ND –

aUnits: ms: millisecond; s:seconds; min: minutes; h: hours; d: days; y: years.

bβ⁺: positron emission; β^–: electron emission; EC: electron capture; ND: not determined

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Chapter 1

4

In general, selenium radioactive isotopes are applied for the production of medical and industrial bromine radioisotopes [10]. Only the ⁷⁹Se, a long-lived β-emitting isotope, is concerned to be severe environmentally hazardous because of its long half-life time of approximately 2.95-3.27×10⁵ years [6, 9, 10]. This radionuclide is mainly created from the fission of ²³⁵U or ²³⁹Pu and approximately 4 ⁷⁹Se atoms are produced per 10000 fission [10].

Significant quantities of ⁷⁹Se only can be found in any associated with irradiated reactor fuel and associated waste nuclear materials because of its low fission yield in nature [3, 11].

1.1.3. The chemistry of selenium

Selenium (Se) has a similar property to sulfur (S) since they are four oxidation states of Se: selenide (2−), selenium (0), selenite (4+), and selenate (6+) [12]. The stability of inorganic selenium species in aqueous solution is exhibits in Fig. 1.1. Selenate (SeO42−

) is dominant at higher redox potentials, whereas at lower redox potentials selenite (SeO32−) and biselenite (HSeO3–) ions dominate. Elemental selenium (Se(0)) probably exists as a solid in association with mineral and organic surfaces within soils and sediments. The reduced forms of selenium are mainly immobilized in ores, whereas the oxidized forms are more mobile and they are toxic species in ecosystems. Selenate (SeO42−) is present as HSeO4− and SeO42− with a pKa of 1.7 [12-14], and selenite (SeO32−) is present as HSeO3− and SeO32− under acidic and alkaline conditions, respectively (pKa1=2.64 and pKa2=8.4) [12-14].

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Chapter 1

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Fig. 1.1 Eh–pH stability diagram of Se at 25 °C, 1.013 bars and activity of Se = 0.13 mM [17].

Selenium is used in variety of industries, including in the production of glass, plastics, medicines, stainless steel, and semiconductors. However, because of its toxicity, an excess concentration of selenium in industrial wastewater causes damage to aqueous environments and living organisms. High levels of selenium are also toxic to humans, with symptoms including gastrointestinal disturbance, discoloration of the skin, decayed teeth, hair or nail loss, nail abnormalities, and changes in peripheral nerves [15, 16]. Therefore, the retention of

79Se and its isotopes is an important issue related long-term radiological safety assessments of solid radioactive waste disposal in geological repositories for preventing environmental contamination and helping to maintain safe ecosystems [16].

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6 1.2. The chemistry of iodine and radioactive iodine 1.2.1. The history and importance of iodine

Iodine (I) was discovered in 1811 by French chemist Bernard Courtois when he sublimed the element from seaweed ash with concentrated sulphuric acid. Because of its characteristic gas color, the origin of the name comes from the Greek word iodes meaning violet [1, 18, 19].

This element is a halogen located in the fifth period of the periodic table in the halogen group.

The distribution of this trace element is quite wide at various concentrations, such as in atmosphere, lithosphere, hydrosphere, and biosphere. However, iodine is attracted a lot of attention on the iodine deficiencies because this element is an essential micronutrient in mammals. World Health Organization (WHO) reported that the iodine deficiency is most prevalent in the world and easily preventable [20, 21]. Iodine deficiencies can attribute to some diseases, especially for metabolic disorders, including thyroid enlargement (goiter), hypothyroidism (underactive thyroid gland), hyperthyroidism (overactive thyroid gland), mental retardation in infants and children whose mothers are iodine-deficient during pregnancy, reproductive damage, and childhood mortality [20, 21].

1.2.2. Iodine and its radioactive isotopes

Iodine has only one stable isotope, ¹²⁷I, with 37 recognized radioactive isotopes and isomers with atomic numbers of 108-144 (Table 1.3). Most of them exhibit very short

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half-lives ranging from minutes to a few hours. Among these radioactive isotopes, the ¹²⁹I and

131I isotope are considered as remarkable environmental risk related to nuclear waste storage or disposal because of its relatively long half-life and high mobility in soil and aqueous system [22-25]. Having a half-life ca. 8.02 days,¹³¹I is mainly created from anthropogenic activities with yield approximately 3% from ²³⁵U and ²³⁹Pu [10, 26, 27]. Based on the previously report, large amounts of ¹³¹I was released from some severe nuclear power plant accidents, such as Chernobyl and Fukushima nuclear power plant explosion [28-30].

Compared with ¹³¹I, ¹²⁹I has the releatively long half life (1.57 × 10⁷ years) and also is the important fission product with a fission yield of 0.9% form ²³⁵U and 1.9% from ²³⁹Pu [10, 31, 32]. As the only naturally occurring radioactive isotope of iodine, ¹²⁹I is produced by cosmic-ray interactions with xenon in the upper atmosphere and by spontaneous fission of

238U in the geosphere [33]. It is reported that the total naturally occurred ¹²⁹I in the surface environment was about 80 kg. Only 5 × 10⁻⁴ kg of ¹²⁹I is existed in the atmosphere [34].

Table 1.3 Data of selenium radioactive isotopes [8, 9, 35].

Isotope Atomatic mass Half life^a Decay mode^b Daughter isotopes

108I 107.94348 36 ms α ¹⁰⁴Sb

109I 108.93815 105 us p ¹⁰⁸Te

110I 109.93524 650 ms β+ ¹¹⁰Te

111I 110.93028 2.5 s β+ ¹¹¹Te

112I 111.92797 3.42 s β+ ¹¹²Te

113I 112.92364 6.6 s β+ ¹¹³Te

114I 113.92185 2.1 s β+ ¹¹⁴Te

115I 114.91805 1.3 min β+ ¹¹⁵Te

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116I 115.91681 2.91 s β+ ¹¹⁶Te

117I 116.91365 2.22 min β+ ¹¹⁷Te

118I 117.913074 13.7 min β+ ¹¹⁸Te

119I 118.91007 19.4 min β+ ¹¹⁹Te

120I 119.910048 81.6 min β+ ¹²⁰Te

121I 120.907367 2.12 h β+ ¹²¹Te

122I 121.907589 3.63 min β+ ¹²²Te

123I 122.905589 13.22 h EC ¹²³Te

124I 123.9062099 4.18 day β+ ¹²⁴Te

125I 124.9046302 59.40 day EC ¹²⁵Te

126I 125.905624 12.93 day β– ¹²⁶Te

128I 127.905809 24.99 min β– ¹²⁸Xe

129I 128.904988 1.54 × 10⁷ year β– ¹²⁹Xe

130I 129.906674 12.36 h β– ¹³⁰Xe

131I 130.9061246 8.02 day β– ¹³¹Xe

132I 131.907997 2.29 h β– ¹³²Xe

133I 132.907797 20.8 h β– ¹³³Xe

134I 133.909744 52.5 min β– ¹³⁴Xe

135I 134.910048 6.57 h β– ¹³⁵Xe

136I 135.91465 83.4 ms β– ¹³⁶Xe

137I 136.917871 24.13 s β– ¹³⁷Xe

138I 137.92235 6.23 s β– ¹³⁸Xe

139I 138.92610 2.28 s β– ¹³⁹Xe

140I 139.93100 860 ms β– ¹⁴⁰Xe

141I 140.93503 430 ms β– ¹⁴¹Xe

142I 141.94018 200 ms β– ¹⁴²Xe

143I 142.94456 100 ms β– ¹⁴³Xe

144I 143.94999 50 ms β– ¹⁴⁴Xe

aUnits: ms: millisecond; s:seconds; min: minutes; h: hours; d: days; y: years.

bβ⁺: positron emission; β^–: electron emission; EC: electron capture; p: proton emission

1.2.3. The chemistry of iodine

Iodine will form compounds with most elements, but is less reactive than the other

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halogens, which displace it from iodide compounds and exhibits some metallic-like properties [1, 12, 18]. Iodine is biophilic and strongly concentrated in seaweeds particularly from which it may be recovered. The inorganic iodine is found at relatively high concentrations in Chilean saltpeter and nitrate-bearing caliche and in brackish waters and brines from oil and gas wells [18].

In water and soil system, the predominant states of inorganic iodine are present as iodide (I^–) and iodate (IO3–) as reduced and oxide forms, respectively [1, 36].When Eh values are high (>1.2V), some high valence inorganic iodine can be present as IO4− and HIO4−[19, 37].

In reducing environments, aqueous iodine usually occurs as the mobile monovalent anion I⁻. Under more oxidizing conditions, iodine is present as the more reactive IO3−, which can lead to retard transportion through interaction with clays and organic matters [10, 38]. Iodide (I^–) is usually exists as in predominant form in an aqueous system, although the iodate (IO3^–) is predicted to be more stable according to the thermodynamic analysis [39]. Specifically, even though I^– and IO3– are both monovalent anions, they show different sorption properties on marine sediments and minerals with the IO3– sorbing to a great degree than I^–[40-42].

When the ¹²⁷I radioactive isotopes entering the environment, this radionuclide exhibits the same behavior as ¹²⁷I, which is stable and ubiquitous in nature. Therefore, it is important to understand the environmental behavior of radioactive iodine (I) by using stable ¹²⁷I in order to evaluate the environmental risks associated with ¹²⁹I. The confinement of radionuclides or

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hazardous elements is an important issue for preventing environmental contamination and helping to maintain safe ecosystems for living organisms.

Fig. 1.2 Eh–pH stability diagram of I at 25 °C, 1.013 bars and activity of I = 0.08 mM [17].

1.3. Radioactive wastes treatment 1.3.1. Radioactive wastes classification

The radioactivity level in nuclear wastes is the main factor to control the handling, treatment and interim storage of wastes. According to the new classification system [43], the main waste classes include exempt, low and intermediate level waste. This classification could be subdivided into short lived and long lived waste, and also high level waste [44].

 The exempt wastes activity levels at or below the clearance levels, which are based on an annual dose to the public of less than 0.01 mSv. For this kind of wastes, there are no special radiological restrictions for treating these wastes.

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 The low and intermediate level wastes have the activity levels above the clearance

levels as described in guidance [43] and thermal power below about 2 kW/m³. Furthermore, the low and intermediate level wastes also can be divided into two groups based on their half-lives and radionuclide concentrations (short lived wastes and long lived wastes). Both of them should be treated in near surface or geological disposal facilities.

 The high level wastes exhibits thermal power above ca. 2 kW/m³ and long lived

radionuclide concentrations exceeding the limitations for short lived waste. These wastes must be disposed in geological disposal facilities.

The collection and segregation of radioactive wastes, reduction of their volume and conditioning into a form suitable for future handling, transportation, storage and disposal should be properly planned. Pertinent technological steps in managing radioactive waste are detailed in Fig. 3 [44].

1.3.2. Radioactive wastes immobilization

As shown in Fig. 1.3, immobilization is the important process for the radioactive waste treatment. The immobilization process could convert the liquid or semi-liquid wastes into the solid form and thus the immobilized hazardous wastes could be disposed more safely and conveniently. This process also can decrease the volume of the wastes via removing the liquid

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phase and have a potential to reduce the mobility of the radionuclides in immobilized wastes after the deposal in environment.

For immobilization of radioactive wastes, several typical matrixes are applied. The long-term storage wastes should use irreversible process which not allows the solid to return to a liquid form. Furthermore, the solidified matrixes should be evaluated toxic target leach ability during disposal. This factor is one of the important considerations in the assessment of a solidification method, as it will strongly influence the radioactive wastes treatment. Low solubility of solidified matrixes improves the safety of radioactive waste management, which further reduces the possibility of radionuclides release. A reversible process may be used for short term storage, or to convert temporarily a liquid into a solid for material handling purposes.

1.3.3 Immobilization materials

Varieties of potential solidification/immobilization materials are available for treating liquid and wet solid wastes. The principal for the selection of any particular material depends on the waste composition and the extent and type of treatment process. The commercially applied immobilization materials have been demonstrated to be viable to various species [44-51].

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Fig. 1.3 The process for radioactive waste managing [44].

The cement is one kind of inorganic materials that have the ability to react with water at ambient conditions to form a hardened matrix. The most common cements are constituted by calcium silicates, such as the Portland cements. Due to its low cost, availability and compatibility with aqueous waste, the application of immobilization radioactive waste via Portland cement began during the early years of the nuclear industry [44-48]. However, some specific wastes, such as organic ion exchange resins or acidic wastes interact with the cement components to inhibit or retard the hydration reaction [44, 45].

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Bitumen is another alternative immobilization matrix for treating low level radioactive wastes [49, 50]. This is a thermoplastic material, which can exhibit mechanically as either a viscous liquid or a solid, based on its temperature. Furthermore, the use of bitumen to solidify low level radioactive wastes has been successfully applied on an industrial for many years in different countries [49].

1.4. Ettringite

1.4.1. Nature occurrence and chemical composition

Ettringite (Ca6Al2(SO4)3(OH)12·26H2O) is the prototype of the hydroxysulfate phase of hydrated cement, which consists of tricalcium aluminate ferrite trisulfate hydrate (Ca6(Al,Fe)2(SO4)3(OH)12·nH2O, AFt). This mineral commonly occurs in the natural alkaline environment, associated with other species like portlandite, gypsum or afwillite [52]. In addition, non-natural of synthesized ettringite also appears as hydrate phase when sulfate exists during the early hydration process of Portland cement [53, 54]. Portland cement is a common material for the immobilization and storage of various hazardous materials and ettringite is supposed to play an important role in hazardous waste immobilization process.

There are compositional varieties in nature ettringite due to substitution of other anions or cations for Ca²⁺, Al³⁺, and SO42– in ettringite. Partial substitution of borate with sulfate and complete substitution of selenate or chromate with sulfate have been peformed[55-57].

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Table 1.4 Ettringite and its related minerals [58, 68-72].

Ettringite Ca6(SO4)3[Al(OH)6]2 · 26H2O Bentorite Ca6(Cr(III),Al)2(SO4)3(OH)12 · 26H2O Buryatite Ca3(Si,Fe,Al)(SO4)[B(OH)4](OH)5 · 12H2O Carraraite Ca3(SO4)[Ge(OH)6](CO3) · 12H2O

Charlesite Ca6(Al,Si)2(SO4)2[B(OH)4](OH,O)12 · 26H2O Hielscherite Ca3Si(SO4)(SO3)(OH)6 · 11H2O

Imayoshiite Ca3Al(CO3)[B(OH)4](OH)6 · 12H2O Jouravskite Ca3Mn⁴⁺(SO4)(CO3)(OH)6 · 12H2O Kottenheimite Ca3Si(SO4)2(OH)6·12H2O

Micheelsenite (Ca,Y)3Al(HPO4,CO3)(CO3)(OH)6 · 12H2O Sturmanite Ca6(Fe³⁺,Al,Mn³⁺)2(SO4)2[B(OH)4](OH)12 · 25H2O Tatarinovite Са3Al(SO4)[B(OH)4](OH)6·12H2O

Thaumasite Ca3(SO4)[Si(OH)6](CO3) · 12H2O

Particularly, ettringite is well-known as an anion-exchanger, and the generic formula for ettringite could be expressed as [M²⁺x(M³⁺)x/3(OH)12][A^n–]x/n·26H2O, where M²⁺ is a divalent cation, including Ca²⁺ Sr²⁺, Pb²⁺, Co²⁺, Zn²⁺, etc. [58-60], M³⁺ is a trivalent cation, including Al³⁺,Ga³⁺, Fe³⁺, Mn³⁺, Cr³⁺, etc. [61, 62] and A^n–is an n-valent anion, including Cl^–, SO42–, SeO42–, AsO43–, etc.[58, 59, 63-67]. Thus, ettringite and its related minerals (Table 1.4) have a potential to be applied as environmental remediation material, scientific understanding of the mechanism of various ions immobilization using ettringite in practical pollutant treatment is required to elucidate for the environmental science and technologies.

1.4.2. Crystal structure

Based on the previously reported structure model of ettringite [58, 73], the unit cell

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mainly consists of column parts {Ca6[Al(OH)6]2･24H2O]}⁶⁺ that are constituted by Al(OH)63–

octahedra and Ca-O8 polyhedra. In the columnar parts, each Ca is eight coordinated by four H2O molecules and four OH^– groups. This means that the Al(OH)63– octahedra and Ca-O8

polyhedra share OH^– groups. SO42– groups and H2O molecules occupy the intercolumn space, which holds the column portions together for structural stability via electrostatic forces (Fig.

1.4) [58, 73]. Additionally, a precise arrangement of the hydrogen bonding network exists in ettringite [74].

In a similar manner to LDHs, high concentrations of anionic species and water molecules are present within the intercolumnar spaces of ettringite through interaction with the high charge density of the columnar regions. This results in strong intercolumnar electrostatic interactions and distinctive hydrophilic properties in the intercolumnar spaces, which can lead to the strong hydrogen bonding network and electrostatic forces for the compact stacking of the column structure. [73, 74]

As cocrystallized material with the hazardous ions, the thermal stability of ettringite is quite important for pollutant ions stabilization. However, the thermal stability of ettringite is quite limited. The behavior of ettringite dehydration and decomposition under thermal condition has been investigated by a number of researchers [74-77]. It is believed that the ettringite will be converted into meta-ettringite, which is partially dehydrated ettringite still maintaining the columnar structure at lower calcination temperatures than 120 °C [76, 77].

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Fig. 1.4 Structure of ettringite. (a) Perpendicular to c-axis shown columns and channels, (b) projection of ettringite structure, (c) expended the dash line regions in (a) and (b) [58, 73].

Amalia et. al. reported that ettringite will be decomposed at lower than 50 °C and could not be as hazardous elements and radionuclides host phase, especially for ⁷⁹Se [75]. However, calcination is a concern for the treating the spent absorbents which have incorporated hazardous elements or radionuclides, such as hot isostatic pressing (HIP) or incorporated in the ceramics, prior to landfilling [78-80]. Thus, the calcination behavior of hazardous elements and radionuclides bearing ettringite should be discussed and investigated for treating hazardous wastes, especially for nuclear wastes treatment.

1.5. The objectives and outline of this thesis

Although trace amount of selenium and iodine could be the crucial nutrient elements to

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human beings, excess uptaking of these elements will also cause some serious diseases. In addition, selenium and iodine radioactive isotopes could be other risks for ecosystems or living organisms. Considering the rarity of ⁷⁹Se and ¹²⁹I in the nature, it is likely that actual human exposures will be extremely low. However, the long-lived nature of ⁷⁹Se and ¹²⁹I and their potential for transport in alkaline geosphere conditions dictate that these radionuclides is of current interest in safety assessments of geological radioactive waste repositories.

The confinement of radionuclides or hazardous elements is an important issue for preventing environmental contamination and helping to maintain safe ecosystems for living organisms.

As one of the major compound forms during cement hydration, ettringite is supposed to play an important role in toxic ions immobilization process in Portland cement because Portland cement is generally applied as a matrix for the immobilization and storage low and intermediate level nuclear wastes and other hazardous wastes. Incorporating of radioactive or toxic ions in ettringite has been investigated by numbers of researchers. However, some aspects are still not completely understood.

Therefore, this study explored the mechanism of SeO32–, SeO42–, I^–, and IO3– coprecipitation with ettringite and the properties of guest anions doped ettringite. Through the ettringite structure, the immobilization of SeO32–, SeO42–, I^–, and IO3– was elaborated to interpret.

Furthermore, the selenate hydrocalumite was proved to be the intermediate to form selenate ettringite. The stability of ettringite was also investigated for evaluation of its ability to

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accommodate radionuclides. However, ettringite exhibits lower stability when this mineral encounters some higher electronegative anions. Because calcination is a concern for the treating the spent absorbents which have incorporated hazardous elements or radionuclides, the calcination behavior of hazardous elements bearing ettringite was discussed and investigated. Finally, some industrial by-products were applied to synthesize glassy-ceramics for incorporating selenate bearing ettringite.

The main research aspects of this thesis are:

(1) The immobilization mechanism of Se oxoanions was evaluated through structural insight into ettringite. SeO32– and SeO42– proved to exhibit different immobilization mechanism. The details of the immobilization mechanism were interpreted via water chemistry, XRD, EXAFS, and FTIR results.

(2) The effect of SO42– on SeO42– immobilization was investigated. When SO42– and SeO42–

coexisted in solution, both oxoanions were immediately coprecipitated with ettringite after adding a Ca source. Without SO42–, SeO42–-substituted hydrocalumite formed as an intermediate which was shown to affect the formation of SeO42––substituted ettringite.

(3) In order to figure out the immobilization performance mechanism of I^– and IO3–

via ettringite, various anions with different ionic radius and valences were selected to co-precipitate with ettringite. According to the different anions' properties and the structure of ettringite, the immobilization mechanism of various anions via ettringite is finally proved to