Dissolution kinetics of nanodispersed gamma-alumina in aqueous solution at different pH: unusual kinetic size effect and formation of a new phase

The dissolution of a technical, nanodispersed gamma-alumina in water was studied at 25 degrees C in the pH range 3.0 < or = pH < or = 11.0. The obtained kinetic dissolution curves showed a distinct pH dependency, whereas only for pH > or = 4.5 the typical behavior of nanodispersed materials could be observed. X-ray powder diffraction analysis and nitrogen adsorption, as well as IR and UV-Raman spectroscopy, were used to characterize the solid material collected during and at the end of each dissolution experiment. As a result the formation of a new aluminum phase-bayerite-could be proven. The analysis of the equilibrium concentration enabled us to determine the solubility constant of the corresponding phase assuming a pH-dependent species distribution. The rate constants of the dissolution process were evaluated using the model of Gibbs free energy of cluster formation, which considers the size effect, among other things. As a result, we could show that the observed maxima in the concentration profiles are due to a size effect of the starting material having a primary particle radius of 10.1 nm.