ALUMINA SURFACE-CHEMISTRY AT 25, 40, AND 60-DEGREES-C

Potentiometric titrations and metal adsorption measurements on gamma-Al2O3 at 25, 40, and 60-degrees-C indicate that alumina surface chemistry is temperature sensitive. The pH(zpc) of gamma-Al2O3 decreases slightly, if at all, from pH 6.45 at 25-degrees-C to 6.32 at 60-degrees-C. Metal binding constants for Cd++ and Pb++, calculated with a constant capacitance model. increase up to 1.5 orders of magnitude between 25-degrees-C and 60-degrees-C. Enthalpies of unidentate Pb and Cd binding are 21.3 and 8 1.8 kj/mol, respectively; these are similar in sign and magnitude to the enthalpies of metal hydrolysis. Because temperature favors the latter, surface coordination is favored by increasing temperature (i.e., and increasing depth in the crust).