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EFFECT OF TANNATE, pH, SAMPLE PREPARATION, AGEING AND TEMPERATURE ON THE FORMATION AND NATURE OF Al OXYHYDROXIDES

¹ Dipartimento di Scienze Animali, Vegetali e dell’Ambiente, Università del Molise, Via De Sanctis s/n, 86100 Campobasso (CB), Italy
² Dipartimento di Scienze del Suolo della Pianta e dell’Ambiente, Università di Napoli Federico II, Via Università, 100, 80055 Portici (NA), Italy
³ Centro Servizi Interdipartimentali di Microscopia, Università del Molise, Via De Sanctis s/n, 86100 Campobasso (CB), Italy

^* E-mail address of corresponding author: colombo{at}unimol.it

The aim of this work was to study the influence of tannate concentration (tannic acid/Al molar ratio [R] of 0.01–0.1), pH (5.0 and 8.0), order of aluminum, hydroxyl, and tannate addition, and prolonged ageing at different temperatures (10 months at 50°C and 5 y at room temperature and further ageing for 3–15 days at 140°C) on the nature and crystallinity of synthetic boehmite. Tannate facilitated the formation of boehmite relative to Al(OH)₃ polymorphs when coprecipitated with Al and also when added to Al(OH)x phases already formed at pH 4.5 or 8.0. However, the organic ligand was more effective in preventing or perturbing the growth of the crystals of boehmite when coprecipitated with Al than when added to soluble Al(OH)_x species or preformed Al precipitates. Boehmite aged at temperatures 50°C typically showed a fibrous morphology. Crystals with a rhombic habit were observed in some treatments after ageing at 140°C. Significant amounts (15–30% C) of tannate were present in the precipitates after prolonged ageing. This tannate was only partly (<50%) oxidized by repeated treatments with H₂O₂. With further ageing at 140°C, the crystallite size of selected samples increased only slightly, suggesting that much of the remaining tannate was present in the internal structure of these samples and not simply adsorbed to their surfaces. Atomic force microscopy observation showed the presence of globular nanoparticles (probably Al tannate precipitates) attached to the elongated crystals of boehmite. Taken together, our results demonstrate that the crystalline phases that formed under mild conditions in the presence of low concentrations (R = 0.01) of foreign ligands have the same structure as boehmite, but with a poorly ordered and defective ion arrangement. At higher ligand concentrations (R 0.05), mixtures of materials are formed having varying degrees of order, particle size and morphology.

Key Words: Atomic Force Microscopy • Boehmite • Crystallite Size • Poorly Ordered Material • Tannic Acid

This article has been cited by other articles:

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