Dissolution and growth of aluminum hydroxide minerals
Subtitle: Atomic force microscopy studies.
Published: 2005
Language: eng
Number of pages: 129
Description: The dissolution and growth of the Al(OH)3 minerals are important in natural and industrial systems. These studies involve manometer scale observations of the features on the surfaces of gibbsite and the structurally similar nordstrandite polymorph. In situ atomic force microscopy (AFM) was used to follow the dissolution of gibbsite in nitric acid at the scale of monolayer steps on the surface. Ex situ AFM was used to observe the basal growth surfaces of nordstrandite crystals at the scale of monolayer steps, and the effects of carboxylates and chloride on this morphology. The steps on both the gibbsite and nordstrandite are composed of straight segments that are crystallographically oriented, primarily either parallel or perpendicular to the 6 edges of the pseudohexagonal crystals. Gibbsite dissolution was facilitated mainly by the retreat of the steps. In addition, the opening of etch pits at screw dislocations was observed. Calculations of the dissolution rate were made based on the observations of step movement, and the rates calculated using only the surface area of the step fronts, 1.8--3.6 x 10 -7 mol m-2 s-2 in 8 M nitric acid, were comparable to the literature values obtained by other methods. On the surfaces of the nordstrandite crystals grown from ethylenediamine solutions, the segments making up the steps are shorter with increasing carboxylate and chloride concentrations in the solutions. 2-dimensional nucleation was the main growth mechanism on the surfaces, with spiral growth also observed. 2-dimensional growth was especially prominent along the reentrant angle of twinned crystals. For nordstrandite crystallized without carboxylate or chloride, nucleation only occurred at a few points, and concentric terraces were formed, whereas with increasing concentrations of these additives, increasingly more random, widespread nucleation occurred. The additives also caused a doubling or bunching of steps. These differences in step morphology can be explained by the adsorption of the carboxylates or chloride at the steps, which affects the step edge surface energy. AFM images of pseudoboehmite precipitated in similar solutions to those used to produce nordstrandite show irregular ribbons 1 or a few boehmite unit layers high, and indicate a lack of long range order.**This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).
Notes: Thesis (Ph.D.)--University of Toronto, 2005.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3012.