To calculate the entropy, the different contributions to the partition function were evaluated using the standard statistical mechanics expressions in the canonical ensemble and the harmonic oscillator and rigid rotor approximation.
AL OH 3 PRECIPITATE FREE
The frequencies were then used to evaluate the zero-point vibrational energy (ZPVE) and the thermal ( T = 298 K) vibrational corrections to the enthalpies and Gibbs free energies within the harmonic oscillator approximation. All structures showed positive force constants for all the normal modes of vibration. To confirm that optimised structures were real minima on the potential energy surfaces, frequency calculations were carried out at the same level of theory. The B3LYP functional has previously been shown to be effective for modelling interactions involving aluminium 17. Herein we have, for the first time, applied density-functional methods of computational chemistry to elucidate reaction intermediates in the formation of HAS A and HAS B and thereby support current solution and solid state data on these critical phases in the biogeochemistry of both aluminium 1 and silicon 2.Īll geometrical optimisations were carried out in aqueous-phase using the Gaussian 09 suite of programmes 10, B3LYP functional 11, 12, 13, 14 and the 6–31++G(d,p) basis set 15, 16.
AL OH 3 PRECIPITATE SERIES
The intermediates in this series of reactions are potentially myriad and their identities are certainly inaccessible through conventional bench-top analytical chemistry. These data have allowed the formulation of potential reaction pathways beginning with monomeric and soluble reaction moieties, Al 3+ (aq) and Si(OH) 4 and finishing with precipitates of HAS A and HAS B. Present understanding of the formation and structures of HAS A and HAS B has, in the main, been obtained from solid state measurements of air-dried, filterable solids 9. Atomic force microscopy was used to show that HAS solids adopt discrete morphologies with HAS A forming flat (1–2 nm) rectangular (up to 170 nm in length) particles while HAS B are also flat (1–2 nm) but discoid (up to 40 nm in diameter) particles 8. This shift from octahedral to tetrahedral geometry is supported by changes in Si coordination from Q 3(3Al), the signature for HAS A, to Q 3(1-2Al) 7. The solid phase of HAS B has an elemental Si:Al ratio of 1.0 and Al(III) is found in tetrahedral (Al IV) and octahedral (Al VI) geometries in equal amounts 7. Filterable solids of HAS A have an elemental Si:Al ratio of 0.50 and their structure is dominated by Si coordinated through three Si-O-Al linkages (Q 33Al) to Al(III) in octahedral geometry (Al VI). Al(OH) 3(s) is a prerequisite to the formation of HAS A which is itself required for the formation of HAS B. HAS A is the predominant HAS formed in solutions in which the initial concentration of Si(OH) 4 is less than or equal to the total aluminium while HAS B predominates where the initial concentration of Si(OH) 4 is at least twice that of aluminium. Two discrete forms of HAS, known as HAS A and HAS B, have been identified 7. The reaction is competitive because Si(OH) 4 competes with the further hydroxylation or autocondensation of Al(OH) 3(s) and its eventual precipitation as gibbsite 6, 7. The mechanism of formation of HAS proceeds via the competitive condensation of Si(OH) 4 across hydroxyl groups on a template of Al(OH) 3(s) 6.
Therefore the unique inorganic chemistry of the formation of HAS has been critical in the non-selection of aluminium in biochemical evolution and it continues to be important in combating the ecotoxicity of aluminium including human exposure to aluminium 5. The formation of HAS is suggested as a basis for the essentiality of silicon in keeping aluminium out of biota 3, 4.
The latter only consists of (i) the autocondensation of silicic acid (Si(OH) 4) in forming silica (nSiO 2 bonds in biota and its limited inorganic chemistry under physiological conditions 2. Silicon’s non-essentiality is reflected both in its non-existent biochemistry, there are no known Si-C, Si-N, Si-O-C…etc. Paradoxically neither element is essential to life and aluminium is inimical to biota. Silicon and aluminium are the second and third most abundant elements of the Earth’s crust 1.
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