Surface characterization of amorphous aluminum oxides

Surface area is a physical property of solids that has been used as an indi cator of reactivity and as an input parameter in chemical surface complexat ion models of ion adsorption. This study was conducted to identify the fact ors that affect the surface area of synthetic AL oxides and to evaluate whe ther surface area is a stable physical property of amorphous Al oxides. Fou r Al oxides were synthesized using four different methodologies. The surfac e area and porosity, especially mesoporosity, were characterized using gas adsorption, mercury intrusion porosimetry, scanning and transmission electr on microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Reactivit y toward adsorption of berate, arsenite, arsenate, and molybdate as a funct ion of solution pH, and ion activity products after aqueous reaction were a lso investigated. Surface areas were found to be affected by various factor s, including aging, drying, heating, reaction in aqueous solution, and conc entration of the starting reagents during synthesis. Aluminum oxide mineral s of widely differing initial surface areas measured in the dry state had s urface areas of comparable magnitude upon reaction in aqueous solution; the initially high surface areas decreased and the initially low surface areas increased to a surface area of 32.4 +/- 2.8 m(2) g(-1). Initial surface ar ea is not a good indicator of chemical reactivity for synthetic amorphous A l oxides. Accurate surface characterization is needed at the time of reacti on if thermodynamically unstable materials are used in adsorption studies.