Pressure induced speciation changes in the aqueous Al³⁺ system

Abstract

We have developed a simple model for incorporating the influence of external pressure and solution pH into a cluster based (i.e. comprising the central Al(3+) cation and nearest neighbor coordinating H2O and OH(-) ligands) 1st principles approach to investigate the hydrolysis equilibria of aqueous Al(3+) monomeric species in high pressure environments such as are found in the Earth's mantle. Our model is demonstrated to reproduce the well documented bulk chemistry of the aqueous Al(3+) system under ambient conditions, namely the system is dominated at low and high pH by the 6-coordinated aqua species and 4 coordinated hydroxide species, respectively, while all remaining species occupy a narrow intermediate pH range. Coupling this model to changes in solution pH is achieved by using [H3O(+)] as a parameter in the definition of the formation equilibrium constants used; the influence of external pressure is evaluated using Planck's equation. This approach predicts that changes in external pressure will induce drastic changes in the aqueous solubility of these species under high pressure conditions and moderate changes at as low as 5 GPa. Finally, some industrial and geochemical implications of this result are discussed.

27Al NMR Chemical Shifts and Relative Stabilities of Aqueous Monomeric Al3+ Hydrolytic Species with Different Coordination Structures

“…Due to that varieties of Al 3+ species may coexist in a narrow pH range and that polymerization and precipitation easily occur at high Al concentrations, it is difficult to use traditional experimental methods such as 27 Al NMR, X-ray diffraction, or potentiometry to determine the existing forms of the intermediate hydrolytic species . Therefore, researchers turned to theoretical methods and have performed a large majority of studies on the coordination structures of the aqueous monomeric Al 3+ hydrolytic species . However, because of the different calculation methods used, there are big differences between the reported dominant forms and their coordination numbers (CNs) for the intermediate hydrolytic products in the literature.…”

“…However, because of the different calculation methods used, there are big differences between the reported dominant forms and their coordination numbers (CNs) for the intermediate hydrolytic products in the literature. For example, for the first hydrolysis product Al­(OH) 2+ , it is reported that its dominant form may be hexa- − or pentacoordinated, ,− or both two coordination structures coexist; , for the second hydrolysis product Al­(OH) 2 + , the hexa-, ,,,, penta-, or tetracoordinated configurations or their coexistence ,, have been reported to be dominant in the literature; for the third hydrolysis product Al­(OH) 3 0 , it is considered that the penta- , or tetracoordinated ,,,,− or both coordination forms , are dominant, and there are even studies suggesting that Al­(OH) 3 0 exists in the tricoordination form. , These inconsistent results have significantly hindered further understanding of the hydrolysis-polymerization mechanisms of Al forms in aqueous solution. In order to solve this problem with the theoretical modeling way, a comprehensive investigation of all possible existing forms of the aqueous monomeric Al 3+ hydrolytic species with a suitable calculation method and solvation model is needed.…”

“…2 Therefore, researchers turned to theoretical methods and have performed a large majority of studies on the coordination structures of the aqueous monomeric Al 3+ hydrolytic species. 3 However, because of the different calculation methods used, there are big differences between the reported dominant forms and their coordination numbers (CNs) for the intermediate hydrolytic products in the literature. For example, for the first hydrolysis product Al(OH) 2+ , it is reported that its dominant form may be hexa-4−9 or pentacoordinated, 2,10−12 or both two coordination structures coexist; 13,14 for the second hydrolysis product Al(OH) 2 + , the hexa-, 6,8,9,15,16 penta-, 12 or tetracoordinated 13 configurations or their coexistence 4,5,14 have been reported to be dominant in the literature; for the third hydrolysis product Al(OH) 3 0 , it is considered that the penta- 6,16 or tetracoordinated 3,9,12,13,17−19 or both coordination forms 5,20 are dominant, and there are even studies suggesting that Al(OH) 3 0 exists in the tricoordination form.…”

Thermodynamic Modeling of Solubility of Corundum in Water at Supercritical Conditions

“…Bogatko et al estimated the partial molar volumes of dissolved Al species at room temperatures, extrapolated equilibrium constants of the hydrolysis reactions to high pressures, and evaluated the change in speciation with pressure. They concluded that the increase in pressure results in the suppression of species with coordination numbers of less than 6.…”

“…Among the charge-neutral species, the fractions of the monohydrate and the dihydrate decrease at the expense of the growth of the trihydrate Al­(OH) 3 ·3H 2 O. As pointed out by the authors, this model is a qualitative one because of the approximations that were used. The effect of the temperature on the speciation at high pressures was not discussed.…”