Adsorption/Desorption Kinetics of MoO2−4onto γ-Al2O3by the Pressure-Jump Technique

✍ Scribed by Chung-Hsin Wu; Cheng-Fang Lin; Shang-Lien Lo; Tatsuya Yasunaga

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 192 KB
• Volume: 208
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.1998.5794

✦ Synopsis

The kinetics and mechanisms of molybdate adsorption/desorption at the ␥-Al 2 O 3 /water interface were studied by using the pressure-jump apparatus with conductivity detection at 298 K. A double relaxation was observed due to the adsorption/desorption process. Adsorption data and triple-layer model (TLM) simulation results suggest the formation of both mono-and bidentate innersphere complexes (SMoO 4 ؊ and S 2 MoO 4 ) at the ␥-Al 2 O 3 surface. The intrinsic equilibrium constants (K eq int ) of the complexes were 10 6.5 M ؊2 and 10 16 M ؊4 , respectively. Based on the relaxation theory and combined results of TLM simulation, a two-step process is proposed. The first step (k 1 , k ؊1 ) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO 4

2؊ and H ؉ . The second step (k 2 , k ؊2 ) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO 4 2؊ from the surface. The values of adsorption and desorption rate constants in the MoO 4 2؊ / ␥-Al 2 O 3 system were determined to be k 1 int ‫؍‬ 5.23 ؋ 10 6 M ؊2 s ؊1 , k -1 int ‫؍‬ 2.41 ؋ 10 s ؊1 , k -2 int ‫؍‬ 1.74 s ؊1 , and k -2 int ‫؍‬ 3.26 ؋ 10 ؊1 s ؊1 . The intrinsic equilibrium constant from kinetic measurements (K kin int ) was 10 6 M ؊2 , which was similar to the intrinsic equilibrium constant (K eq int ), 10 6.5 M ؊2 , from the equilibrium studies.