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

• Published in: Journal of colloid and interface science 1998-12, Vol.208 (2), p.430-438
• Main Authors: Wu, Chung-Hsin, Lin, Cheng-Fang, Lo, Shang-Lien, Yasunaga, Tatsuya
• Format: Article
• Language: English
• Subjects: intrinsic adsorption/desorption rate constant; MoO2−4; pressure-jump; TLM; γ-Al2O3
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1006/jcis.1998.5794

The kinetics and mechanisms of molybdate adsorption/desorption at the γ-Al2O3/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 inner-sphere complexes (SMoO−4and S2MoO4) at the γ-Al2O3surface. The intrinsic equilibrium constants (Kinteq) of the complexes were 106.5M−2and 1016M−4, respectively. Based on the relaxation theory and combined results of TLM simulation, a two-step process is proposed. The first step (k1,k−1) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO2−4and H+. The second step (k2,k−2) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO2−4from the surface. The values of adsorption and desorption rate constants in the MoO2−4/γ-Al2O3system were determined to bekint1= 5.23 × 106M−2s−1,kint–1= 2.41 × 10 s−1,kint–2= 1.74 s−1, andkint–2= 3.26 × 10−1s−1. The intrinsic equilibrium constant from kinetic measurements (Kintkin) was 106M−2, which was similar to the intrinsic equilibrium constant (Kinteq), 106.5M−2, from the equilibrium studies.