Adsorption/Desorption Kinetics of MoO2-4 onto gamma-Al2O3 by the Pressure-Jump Technique

The kinetics and mechanisms of molybdate adsorption/desorption at the gamma-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-4 and S2MoO4) at the gamma-Al2O3 surface. The intrinsic equilibrium constants (Kinteq) 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 (k1, k-1) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO2-4 and H+. The second step (k2, k-2) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO2-4 from the surface. The values of adsorption and desorption rate constants in the MoO2-4/gamma-Al2O3 system were determined to be kint1 = 5.23 x 10(6) M-2 s-1, kint-1 = 2.41 x 10 s-1, kint-2 = 1.74 s-1, and kint-2 = 3.26 x 10(-1) s-1. The intrinsic equilibrium constant from kinetic measurements (Kintkin) was 10(6) M-2, which was similar to the intrinsic equilibrium constant (Kinteq), 10(6.5) M-2, from the equilibrium studies. Copyright 1998 Academic Press.