Kinetics of sulfate and selenate adsorption/desorption onto γ-Al2O3 by the pressure-jump technique

Sulfate and selenate adsorption/desorption on gamma-Al2O3 in aqueous suspension were investigated thermodynamically and kinetically. The triple-layer model fit the experimental data well when adsorbed sulfate and selenate were assumed to form outer-sphere complexes (SOH2+-SO42- and SOH2+-SeO42-). The intrinsic equilibrium constants (K-eq(int)) of the complex were 10(10.9) M-2 for SOH2+-SO42- and 10(10.6) M-2 for SOH2+-SeO42-. Pressure-jump relaxation was used to determine the kinetics of SO42- and SeO42- adsorption/desorption on gamma-Al2O3. A single relaxation was observed due to the adsorption/desorption processes. Based on the relaxation theory and combined results of TLM simulations, a single-step process is proposed. The intrinsic values of the adsorption and desorption rate constants were determined to be k(1)(int) = 2.7 x 10(8) M(-2)s(-1) and k(-1)(int) = 1.1 x 10(-2) s(-1) in the SO42-/gamma-Al2O3 system and k(1)(int) = 4.3 x 10(8) M-2 s(-1) and k(-1)(int) = 4.4 x 10(-2) s(-1) in the SO42-/gamma-Al2O3 system. The intrinsic equilibrium constants from kinetic measurements (k(kin)(int)) were 10(10.4) M-2 for SO42- and 10(10) M-2 for SeO42-, which were similar to the intrinsic equilibrium constants (K-eq(int)) 10(10.9) M-2 for SO42- and 10(10.6) M-2 for SeO42-, from the equilibrium studies.