Formation and dissolution kinetics of Al(OH) sub 3 (s) in synthetic freshwater solutions

The precipitation of Al in aqueous solutions can be described as a two-step process. When acidic inorganic Al solutions (pH 4.5) were titrated with NaOH to pH levels between 5.5 and 6.0, an amorphous Al (OH){sub 3} (s) phase was formed instantaneously. During the first 5 min, the apparent half time for the reduction in dissolved Al species (t {1/2}) was 0.162 {plus minus} 0.07 hours (n = 4). The decrease of dissolved Al species continued during the following 24 hours, but at a far slower rate. The highest precipitation rates were found in the solution of highest pH, and at approximately identical pH, the highest rate was found in the solution of highest temperature. The dissolution of amorphous Al(OH){sub 3} (s) can also be described as a two-step process. When the amorphous Al solutions (pH between 5.5 and 6.0) were acidified to pH about 4.5 (HCl), the initial dissolution was relatively fast, but not as instantaneous as in the first step of precipitation. After 24 hours of storage 40 to 50% of the original Al(OH){sub 3} (s) remained in the solid phase. This is explained by the presence of a far lower dissolution rate in a second step, i.e., the remaining Al(OH){sub 3} (s) was less soluble. Results suggest that a total dissolution of Al (97%) will occur after 15.3 {plus minus} 4.9 days. The implication of these findings are discussed with reference to the current geochemical models pretending to simulate and predict aluminum chemistry in soils and surface runoff.