Adsorption/desorption kinetics of MoO{sub 4}{sup 2{minus}} onto {gamma}-Al{sub 2}O{sub 3} by the pressure-jump technique
- Natural Taiwan Univ., Taipei (Taiwan, Province of China). Graduate Inst. of Environmental Engineering
The kinetics and mechanisms of molybdate adsorption/desorption at the {gamma}-Al{sub 2}O{sub 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 inner-sphere complexes (SMoO{sub 4}{sup {minus}} and S{sub 2}MOO{sub 4}) at the {gamma}-Al{sub 2}O{sub 3} surface. The intrinsic equilibrium constants (K{sub eq}{sup int}) of the complexes were 10{sup 6.5} M{sup {minus}2} and 10{sup 16} M{sup {minus}4}, respectively. Based on the relaxation theory and combined results of TLM simulation, a two-step process is proposed. The first step (k{sub 1}, k{sub {minus}1}) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO{sub 4}{sup 2{minus}} and H{sup +}. The second step(k{sub 2}, k{sub {minus}2}) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO{sub 4}{sup 2{minus}} from the surface. The values of adsorption and desorption rate constants in the MoO{sub 4}{sup 2{minus}}/{gamma}-Al{sub 2}O{sub 3} system were determined to be k{sub 1}{sup int} = 5.23 {times} 10{sup 6} M{sup {minus}2} s{sup {minus}1}, k{sub {minus}1}{sup int} = 2.41 {times} 10 s{sup {minus}1}, k{sub {minus}2}{sup int} = 1.74 s{sup {minus}1}, and k{sub {minus}2}{sup int} = 3.26 {times} 10{sup {minus}1} s{sup {minus}1}. The intrinsic equilibrium constant from kinetic measurements (K{sub kin}{sup int}) was 10{sup 6} M{sup {minus}2}, which was similar to the intrinsic equilibrium constant (K{sub eq}{sup int}), 10{sup 6.5} M{sup {minus}2}, from the equilibrium studies.
- OSTI ID:
- 316296
- Journal Information:
- Journal of Colloid and Interface Science, Vol. 208, Issue 2; Other Information: PBD: 15 Dec 1998
- Country of Publication:
- United States
- Language:
- English
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