Isomorphic capacity

Titanite, CaTiSi05 (C2/c), may incorporate Na+, REE3+ and minor ACT in the Ca site, and Fe3+, Al3+, Nb5+ in the Ti site. The ThOz content of natural titanite may reach 0.28 wt% (Hayward 1988). Measured isomorphic capacities of titanite with respect to U4+, Pu4+, Pu3+, Hf4+, and Gd3+ are (in atoms per formula unit) 0.02-0.05 (Vance et al. 2000) or 0.07 (Stefanovsky et al. 2000b) 0.02 0.05 0.5 0.3 (Vance et al. 2000) or 0.25 (Stefanovsky et al. 2000b), respectively. Due to limited solubility of ACTs and REEs in the structure, titanite is preferably considered as a host phase for these elements when their content is low, such as in titanite-based glass-ceramics developed for Canadian waste (Hayward 1988). 

Nikonov, B. S., Omelianenko, B. I. Lapina, M. I. 2000b. Isomorphic capacity of synthetic sphene with respect to Gd and U. Materials Research Society Symposium Proceedings, 608, 407-412. 

No single waste form satisfies completely all of these requirements. However, the key requirements are a high isomorphic capacity of host phase with respect to actinide (with neutron absorbers as necessary), high chemical durability -... 

Zirconate pyrochlores have a low isomorphic capacity with respect tetravalent actinides. However the amount of isomorphic substitution can be increased by coupled substitution of Ca, according to the scheme 2REE = An" -i- Ca. Production of single phase ceramic can be attained only for actinide waste streams with rather simple chemical compositions. In the case of a complex waste stream composition, extra phases occur [129]. Finally, zirconate matrices require high temperatures for their synthesis. In order to prepare REE-zirconates from oxide mixtures, sintering at 1500-1600 for about 50 hours is required... 

Thorium phosphate-diphosphate Th4(P04)4P207 (TPD, Pcam) is an actinide host phase due to its very high chemical durability and radiation stability [165-167]. TPD is synthesized by drying of thorium nitrate and phosphorus acid or ammonium phosphate solution, cold pressing at 300-800 MPa, and sintering of pellets at 1100-1250 for 10-30 hours. Th" in the TPD structure may be replaced by other tetravalent actinides but its isomorphic capacity is reduced with decreasing cationic radii in the following sequence > Np" > Pu". ... 

Specific Heats of Solid Mixtures.—The specific heat of a homogeneous solid mixture of solid components is not usually additively composed of the specific heats of the latter. W. Spring (1886) found that the total heat capacity of alloys of lead and tin was always greater than the sum of those of the components, but above the melting-point the two were equal. A. Bogojawlensky and N. Winogradoff (1908) find, however, that the heat capacities of the isomorphous mixtures ... 

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

Since cations are adsorbed electrostatically not only due to the permanent structural charge, a0, (caused by isomorphic substitution) but also due to the proton charge, oh, (the charge established because of binding or dissociating protons -see Chapter 3.2) the ion exchange capacity is pH-dependent (it increases with pH). Furthermore, the experimentally determined capacity may include inner-spherically bound cations. 

Robertson et al. (1954) analyzed two kaolinites in detail and concluded that Fe was present in the octahedral sheet and that there was sufficient isomorphous substitution to account for the cation exchange capacity (Table LXIV). These clays were not pure and it was necessary to make a number of assumptions in order to obtain these results. 

It is assumed that some exchange is due to isomorphous substitution but this has not been proven. Schofield and Samson (1953) calculated that only one Al3+ need replace one Si4+ in 400 unit cells to afford an exchange capacity of 2 mequiv./lOOg. There is enough excess Al3+ in most kaolinites to account for 10 times this exchange capacity. Thus it appears likely that most of the excess Al3+ does not substitute in the tetrahedral sheet. The iron-rich kaolinite described by Kunze and Bradley (1964)has an exchange capacity of 60 mequiv./lOO g however, it is likely that much of this is due to the presence of iron oxides. 

Schematic 1. The structure of 2 1 layered silicates. M is a monovalent charge compensating cation in the interlayer and x is thedegree of isomorphous substitution, which for the silicates of interest is between 0.5 and 1.3. The degree of isomorphous substitution is also expressed as a cation exchange capacity (CEC) and is measured in milli-equivalents/g.

Cation exchange capacity (CEC)—specific surface. CEC varies from 1 to 10 meq/100 g or cmolc kg"1, while specific surface varies from 10 to 20 m2 g1. CEC is pH dependent and therefore not due to isomorphous substitution... 

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