Formation Mechanism of Aluminum Hydroxide Polymorphs

A. Violante1 and P. M. Huang2
1 Dipartimento di Scienze Chimico-Agrarie, Università di Napoli “Federico II” 80055 Portici, Napoli, Italy
2 Department of Soil Science, University of Saskatchewan, Saskatoon Saskatchewan, Canada S7N 0W0
Boehmite formed at room temperature and one atmosphere pressure is a poorly crystalline, highly defective and or fine size AI-oxyhydroxide, usually called pseudoboehmite.

Abstract: Substantial studies have been carried out to investigate the mechanism of the formation of Al(OH)3 polymorphs. The influence of the nature of Al precipitation products on the formation of Al(OH)3 polymorphs still remains obscure. In this study, X-ray diffraction, infrared and thermal analyses, and electron microscopic observations of the Al precipitates formed at the initial pH 8.2 and at a citric acid/Al molar ratio of 0.01 and aged for 3 hr to 60 days revealed that the transformation from the initially formed noncrystalline materials to pseudoboehmite occurred through the formation of intermediate materials with various degrees of ordering and sizes of particles that apparently had a wide range of solubility. By increasing the pH of the suspension of precipitation products of Al to 10.0 after 3 hr and 3, 11, 31, and 60 days or longer, the crystalline precipitation products were hayerite, nordstrandite and bayerite, nordstrandite and pseudoboehmite, pseudoboehmite and gibbsite, and pseudoboehmite, respectively. This work shows evidence that, as the nature of the starting Al precipitates changed, the rate of their dissolution apparently changed, and various Al(OH)3 polymorphs consequently formed. Therefore, the data substantiate the hypothesis that the mechanism of the formation of an Al(OH)3 polymorph is determined by the rate of its nucleation, which is, in turn, influenced by the rate of dissolution of the noncrystalline or poorly ordered Al-oxides initially formed.

Key Words: Al(OH)3 polymorphs • Bayerite • Gibbsite • Noncrystalline materials • Nordstrandite • Pseudoboehmite

Clays and Clay Minerals; October 1993 v. 41; no. 5; p. 590-597; DOI: 10.1346/CCMN.1993.0410509
© 1993, The Clay Minerals Society
Clay Minerals Society (www.clays.org)