Adsorption/Desorption Kinetics of MoO4

The kinetics and mechanisms of molybdate adsorption/desorption at the g-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 monoand bidentate innersphere complexes (SMoO4 2 and S2MoO4) at the g-Al2O3 surface. The intrinsic equilibrium constants (Keq ) of the complexes were 10 M and 10 M, respectively. Based on the relaxation theory and combined results of TLM simulation, a two-step process is proposed. The first step (k1, k21) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO4 22 and H. The second step (k2, k22) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO4 22 from the surface. The values of adsorption and desorption rate constants in the MoO4 / g-Al2O3 system were determined to be k1 int 5 5.23 3 10 M s, k–1 int 5 2.41 3 10 s, k–2 int 5 1.74 s, and k–2 int 5 3.26 3 10 s. The intrinsic equilibrium constant from kinetic measurements (Kkin int ) was 10 M, which was similar to the intrinsic equilibrium constant (Keq ), 10 M, from the equilibrium studies. © 1998