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a USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507
b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
Corresponding author (sgoldberg{at}ussl.ars.usda.gov)
Surface area is a physical property of solids that has been used as an indicator of reactivity and as an input parameter in chemical surface complexation models of ion adsorption. This study was conducted to identify the factors that affect the surface area of synthetic Al oxides and to evaluate whether surface area is a stable physical property of amorphous Al oxides. Four Al oxides were synthesized using four different methodologies. The surface area and porosity, especially mesoporosity, were characterized using gas adsorption, mercury intrusion porosimetry, scanning and transmission electron microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Reactivity toward adsorption of borate, arsenite, arsenate, and molybdate as a function of solution pH, and ion activity products after aqueous reaction were also investigated. Surface areas were found to be affected by various factors, including aging, drying, heating, reaction in aqueous solution, and concentration of the starting reagents during synthesis. Aluminum oxide minerals of widely differing initial surface areas measured in the dry state had surface 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 m2 g-1. Initial surface area is not a good indicator of chemical reactivity for synthetic amorphous Al oxides. Accurate surface characterization is needed at the time of reaction if thermodynamically unstable materials are used in adsorption studies.
Abbreviations: BET, Brunauer–Emmett–Teller • DDIW, doubly deionized water • EGME, ethylene glycol monoethyl ether • IAP, ion activity product • NMR, nuclear magnetic resonance • SEM, scanning electron microscope • TEM, transmission electron microscope
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