Thermodynamic data silicate species

The coal ash deposits on boiler tubes have frequently a separate zone structure with a sulphate rich layer up to 2 mm thick under the matrix of sintered ash (3 ). The outer layer is porous and it constitutes a pathway for the enrichment of alkali-metals in the deposit layer next to tube suface. The diffusable species may be sulphate, chloride, oxide or hydroxide, but the thermodynamic data W and the results of deposition measurements in coal fired boilers (Figure 3) suggest that sodium and potassium sulphates are the principal vapour species which diffuse through a porous matrix of silicate ash deposit. 

A central issue in the attempt to establish a reliable database is the requirement of critically evaluated thermodynamic data for several key species. One such pivotal element is aluminum, which has an extensive literature of solubility and thermochemical data from which to choose, for each of the aqueous species or complexes. The aluminum species are fundamental to the calculation of solubility and reaction state with respect to many silicates and aluminum oxides and hydroxides and are principal components in numerous surface chemical reactions in the environment. Two key chapters in this volume address this fundamental problem Apps and Neil give a critical evaluation of the data for the aluminum system and Hem and Roberson present the kinetic mechanisms for hydrolysis of aluminum species. 

Table 2. Summary of Revised Thermodynamic Data. IV. Silicate Species...

There also remains a considerable degree of uncertainty in the aqueous thermodynamic data for a number of radionuclides of interest in HLW management. The Nuclear Energy Agency (NBA) Thermodynamic Data Base Project has produced volumes on U (Wanner Forest, 1992), Am (Silva et al., 1995) and Tc (Rard et al., 1999). An additional volume in Np and Pu has only recently become available (Lemire et al., 2001), and was not used in this study. And even for those volumes published, a number of key minerals and species may not be well constrained. For example, the NEA database for U does not include data for many key U-silicate minerals such as uranophane, haiweeite, and soddyite, and the equilibrium constant for the neutral aqueous species 1102(011)2 is only bounded at an upper limit. 

Dissolution or precipitation reactions are generally slower than reactions among dissolved species, but it is quite difficult to generalize about rates of precipitation and dissolution. There is a lack of data concerning many geo-chemically important solid-solution reactions kinetic factors will be discussed later (Chapter 13). Frequently, the solid phase formed incipiently is metastable with respect to a thermodynamically stable solid phase. Examples are provided by the occurrence under certain conditions of aragonite instead of stable calcite or by the quartz oversaturation of most natural waters. This oversaturation occurs because the rate of attainment of equilibrium between silicic acid and quartz is extremely slow. 

We had taken this fact into account in performing the calculations for the thermodynamic quantities. The model adopted for this system was the basal plane of the kaolinite covered with three or five monolayers of octadecylammonium cations in their trans conformation closely packed with each other. This implies the existence of atomic sheets in the modifying layer as well as in the silicate layer nearest to the surface the deeper silicate layers were included into the calculations in the bulk approximation. The data [11] were used for the BC potential coefficients employed for the interaction between the H and C atoms of the adsorbate molecule and H and C atoms of the modifier alkyl chain for the coefficients describing the interaction between the alkane atomic species and the ions constituting the silicate lattice, we used the values reported in [13]. The coefficients for the interaction with the N atom in the modifying layer were estimated using the quantum-... 

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