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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.

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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 IO3coprecipitation 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

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relate to the subjected anions' hydration ability and ionic radius.

(4) The structural transformation of ettringite has been evidenced based on the results of X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and extended X-ray adsorption fine structure (EXAFS). The structural transformation is caused by the electrostatic repulsion between sulfate and selenate. However, the atomic arrangement of columnar parts of selenate ettringite is still maintained.

(5) The thermal stability of SeO42– bearing ettringite was investigated. The reduction of SeO42− to SeO32− and the changes of coordination of Se oxoanions were observed under high-temperature calcination. Furthermore, calcination of SeO42– bearing ettringite at 800 °C much improved the stability of selenium.

(6) Selenate-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 800 °C, the amorphous mixture was converted to glass-ceramics. The synthesized ceramics exhibited excellent behavior for immobilization of selenate. Stabilization mechanism is revealed by X-ray photoelectron spectroscopy (XPS).


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Chapter 2

Selenite and selenate uptaken in ettringite: Immobilization

mechanisms, coordination chemistry, and insights from structure

Chapter 2