The adsorption mechanism of trivalent Ga and In onto g-Al 2 O 3 examined the sorption mechanisms of anions at the water/ was investigated using a triple-layer model simulation and presgoethite interface. The results of these works have indicated sure-jump technique. Bidentate Ga 3/ and In 3/ and monodentate that inner-sphere surface complexes are formed between the GaOH 2/ /InOH 2/ are the most likely surface species responsible divalent ions and oxide surface hydroxyl sites. The sorption for Ga(III)/In(III) adsorption. Sorption of Ga(III) and In(III) pathways are postulated as a two-step mechanism with a can be interpreted as an associative process. The adsorption pathmetal ion attached first, followed by the release of a proton way is a two-step mechanism: proton release from surface hydroxyl from the surface hydroxyl group. Furthermore, the intrinsic group(s) followed by coordination of Ga(III)/In(III) species to adsorption rate constants of divalent metal ions obtained the depronated site(s). Intrinsic adsorption rate constants cannot be estimated with a liner free-energy relationship between the from pressure-jump kinetic experiments are closely related adsorption rate constant and the rate of water exchange, which is to the water exchange rate of metal ions. Hence, the intrinsic developed solely based on the dissociative sorption mechanism of adsorption rate constants can be calculated from the water divalent ions. ᭧ 1997 Academic Press exchange rate of the aquo metal ions based on the linear Key Words: Ga(III); In(III); g-Al 2 O 3 ; adsorption; water exfree-energy relationship (LFER) of Hachiya (7). Other inchange rate; linear free energy; intrinsic adsorption rate constants. vestigators (11, 12), however, report that the sorption mechanism of Cr(III) exhibits different reaction steps from those postulated by Hachiya and Hayes. The intrinsic adsorption