Hydrolysis-precipitation studies of aluminum (III) solutions. 2. A kinetic study and model

The hydrolysis-precipitation process of Al(OH)3 from acidified aluminum nitrate and chloride solutions was studied with a homogeneous alkali injection technique. In these studies the total Al(III) concentration (CAl) varied from 5 × 10−5 to 0.5 M, the ionic strength from about 10−4 (no added salt) to 2.8 M; and the temperature from 25 to 90°C. Continuous and discontinuous kinetic experiments with both alkali and acid suggest that different processes occur below and above an OH/Al ratio = 2.5 in solutions with cAl3+ > 5 × 10−4 M. At very low concentration the reaction proceeds solely via monomeric species. Light scattering measurements are used to determine the average degree of polymerization, Nz. It is observed that over the range 0 < OH/Al < 2.5 the average degree of polymerization changes in the order 0 < Nz < 15. The reactivity of these polynuclear complexes with acid suggests that the hydroxyl ions form bridges between the Al3+ cations up to an OH/Al ratio of 2.5. A model, which is based upon the structure of the solid, is postulated for the hydrolysis process. This model appears to be in qualitative agreement with experiment.