Extent of Isomorphous Substitution

When other metals (M) substitute for Fe in the structure of an Fe oxide the mole ratio of substitution is given by Mt/(Mt + Fet)(mol/mol), where Mt and Fet (t = total) are expressed in mol. Fe and other metals present at the surface of the iron oxide or in separate phases must be determined separately to correct the extent of substitution. Ferrihydrite as a separate phase can be selectively dissolved an with acid oxalate solution (see p. 50). This treatment also dissolves any separate Mn or Cr oxides. Alternatively, a short extraction (30 min, 25 °C) with 0.4 M HCl removes adsorbed surface species this method is useful if the solubility of the substituting ion in acid oxalate solution is not known or if the iron oxide under consideration (for example magnetite) is soluble in acid oxalate solution. The total Fet and Mt have then to be corrected for the oxalate soluble Fe and M. 

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The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

The end product of the dehydroxylation of pure phases is, in all cases, hematite, but with lepidocrocite, maghemite occurs as an intermediate phase. The amount of water in stoichiometric FeOOH is 10.4 g kg , but adsorbed water may increase the overall amount released. Thermal dehydroxylation of the different forms of FeOOH (followed by DTA or TG) takes place at widely varying temperatures (140-500 °C) depending on the nature of the compound, its crystallinity, the extent of isomorphous substitution and any chemical impurities (see Fig. 7.18). Sometimes the conversion temperature is taken from thermal analysis data (e. g. DTA), but because of the dynamic nature of the thermoanalysis methods, the temperature of the endothermic peak is usually higher than the equilibrium temperature of conversion. 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

The extent of isomorphic substitution is dictated by the nature of the clay, and this is expressed by the cation exchange capacity (CEC). The CEC of a clay is the number of charges on the clay (expressed in coulombs pa- kilogram) which can be replaced in solution. It is typically in the range of 10 to 10 C/kg and for a given clay is not sensitive to variables such as pH or concentration of the electrolyte in solution. 

The disparity in size of the aluminate and the silicate tetrahedra must be the reason why, at least for some frameworks, the range of Si/Al ratios, and therefore the extent of the post-synthesis isomorphous substitution of Al for Si is limited (27). For boron, with the ionic radius of 0.23 A as compared with 0.51 A for aluminium, the disparity in size is even greater (2). Quantum chemical calculations predict that the tetrahedral coordination of aluminium is favoured in comparison with BO4 groupings (32.33). An attempt to insert boron into the framework of ferrierite (34). a structurally related zeolite, was unsuccessful. 

Aluminium can be isomorphously substituted for silicon in the framework of zeolite Y by hydrothermal treatment of the dealuminated (ultrastabilised) zeolite with aqueous solutions of strong bases at elevated temperatures. The extent and efficiency of the reaction depend on the temperature, duration of treatment and especially on the kind and concentration of the basic solution. The degree of crystallinity and the thermal stability of the products are primarily controlled by the composition of the parent material. 29Si and 27A1 magic-angle-spinning NMR (MAS NMR) indicates that the extent of realumination is determined by the number of available tetrahedral Si(OAl) sites. 

Ainsworth et al. (1994) observed that oxide aging did not cause hysteresis of trace element cation sorption-desorption. Aging the hydrous ferric oxide with trace elements cations resulted in hysteresis with Cd and Cu, but little hysteresis was observed with Pb. The extent of reversibility with aging for Co, Cd, and Pb was inversely proportional to the ionic radius of the ions (i.e., Co < Cd < Pb). The authors attibuted the hysteresis to Co and Cd incorporation into a recrystallizing solid (probably goethite) via isomorphic substitution, not to micropore diffusion. 

The dewaxing performance of ZSM-5 zeolite was shown to depend on the A1 content [168] and of crystallite size [168,169]. Moreover, isomorphous substitution of A1 by other trivalent cations (e.g. B, Fe, Ga) leading to weaker acid sites decreased the activity though it significantly increased the dewaxing selectivity by reducing the extent of secondary cracking [170],... 

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