Al-hydroxy complexes

Taylor, R.M. (1988) Proposed mechanism for the formation of soluble Si-Al and Fe(lll)-Al hydroxy complexes in soils. Geoderma 42 65-77... 

Arnorsson, S. Andresdottir, A. 1999. The dissociation constants of Al-hydroxy complexes at 0-350 °C and Psat. In Proceedings of the Sixth International Symposium on Geochemistry of the Earth s Surface, Reykjavik, 425-428. 

Figure 7.8 Solubility of (a) amorphous AKOHlj, = 10 , and (b) gibbsite [Al(OH)3], K p = 10 , as a function of pH at 25°C. Also shown are lines indicating the solubility contributions of Al and individual Al-hydroxy complexes.

In the equilibrium constant, the superscript asterisk indicates that the dissolution reaction is written in terms of protons. The subscript following K (zero in this case) is the number of hydroxyl ions associated with AF+ in the reaction. According to Nordstrom et al. (1990), Kq = 10. Reactions similar to (7.35) can be written in which kaolinite is dissolved to form each of the aluminum hydroxy complexes listed in mass-balance equation (7.22) for total aluminum. These reactions can be generated by successively adding the cumulative reactions for the Al-hydroxy complexes—Eqs. (7.23) to (7.26)—to Eq. (7.35). For example, adding Eqs. (7.35) and (7.23) and their log values for 25°C, we obtain ... 

The effect of OH and O 2 on the reactivity of complexes deserves special mention. It was found that some hydroxy complexes—e.g., Al(OH)4 and Zn(OH)4 2, are extremely inert to attack by e aq (11). Since no orbitals are involved in these cases, it must be concluded that OH is an extremely poor bridge for electrons. Hydroxy complexes which still contain water in their inner sphere, like cuprate (II), chromate (III), or plumbate(II) do not differ as much from their corresponding aquo complexes. The inhibitory effect of O-2 on the reactivity of the central atom is not sufficient to suppress the reactivity of the metal ions in their higher states of oxidation. Thus we find that Mn04 or Cr207 2 react at diffusion-controlled rates (122). 

The substitution of the mass action expressions for each species into the MBE for aluminium results in equation (5.42). In this example there is now only one unknown in the equation, Al3+, so the value Al3+ can be calculated for any given value of [Al]x. Once Al3+ has been determined the amounts of the hydroxy complexes can be determined by substitution of Al3+ into the formation equation of the relevant species ... 

The rebound mechanism, though in a modified version, has been recently supported by theoretical calculations of KIF using the density functional theory (Yoshizawa et al., 2000). The calculations demonstrate that the transition state for the H-atom abstraction from ethane involves a linear [FeO.H...C] array a resultant radical species with a spin density of nearly one is bound to an iron-hydroxy complex, followed by recombination and release of product ethanol. According to the calculation of the reaction energy profile, the carbon radical species is not a stable reaction intermediate with a finite lifetime. The calculated KIF at 300 K is in the range of 7-13 in accord with experimental data and is predicted to be significantly dependent on temperature and substituents. It was also shown from femtosecond dynamic calculations in the FeOVCH4 system that the direct abstraction mechanism can occur in 100-200 fs. 

The use of Al(III) complexes as catalysts in Lewis acid mediated reactions has been known for years. However, recent years have witnessed interesting developments in this area with the use of ingeiuously designed neutral tri-coordinate Al(lll) chelates. Representative examples involving such chelates as catalysts include (1) asymmetric acyl halide-aldehyde cyclocondensations, " (2) asymmetric Meerwein-Schmidt-Ponndorf-Verley reduction of prochiral ketones, (3) aldol transfer reactions and (4) asymmetric rearrangement of a-amino aldehydes to access optically active a-hydroxy ketones. It is important to point out that, in most cases, the use of a chelating ligand appears critical for effective catalytic activity and enantioselectivity. 

Figure 5. Stahility constants of hi hydroxy complexes of divalent metal cations of the first transition series. Wagman et al. (36) (X) Baes and Mesmer (23) ( JYatsimirskii and VasiVev (11) (A) Smith and Martell (16).

Sorption in most soils attains a maximum when the neutral hydroxy complex of uranium is at a maximum. However, at pH 6 and above, and in the presence of high carbonate or hydroxide concentrations, uranium may form anionic complexes such as [U02(0H)4]. The mobility of anionic uranium complexes in soil is dependent upon the nature of the soil. For example, the decrease in sorption in soil with little anion-exchange capacity may result in increased mobility however, increased sorption in soil with high anion-exchange may result in decreased mobility (Allard et al. 1982 Ames et al. 1982 Brookins et al. 1993 Ho and Doern 1985 Hsi and Langmuir 1985 Tichnor 1994). 

Good summaries of accepted experimental techniques can be found in the references that are cited for individual radionuclides in the sections below. Nitsche (1991) provides a useful general summary of the principles and techniques of solubility studies. A large number of techniques have been used to characterize the aqueous speciation of radionuclides. These include poten-tiometric, optical absorbance, and vibrational spectroscopy. Silva and Nitsche (1995) summarize the use of conventional optical absorption and laser-based photothermal spectroscopy for detection and characterization of solution species and provide an extensive citation list. A recent review of the uses of Raman and infrared spectroscopy to distinguish various uranyl hydroxy complexes is given by Runde et al. (2002b). 

Non-exchangeable acidity. Bound and that is not displaced, or is extremely slowly displaced, into solution by a concentrated neutral salt, usually l.O M KCl. This form is associated with weak acid groups on humus, organically complexed Al, and Al-hydroxy cations strongly retained at mineral surfaces. 

For the formation of hydroxyl and proton complexes, the story is different. Aluminum, for example, may exist in solution dominantly as various hydroxy-complexes such as Al(OH)2 andAl(OH)2+. Obviously, then, the charge on much of the aluminum in solution is less than +3. But the OH- contribution to the charge balance is determined from the p, which reflects only the free OH-, not the complexed OH-. In acid solutions, free OH- are completely insignificant, but the complexed OH- is not. In very alkaline solutions, the story is similar, but it is the role of H+ which becomes important. It is this factor which causes the difference between stoichiometric and speciated charge balances. 

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