RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Sulfate adsorption/desorption on goethite (FeOOH) in aqueous suspension was investigated thermodynamically and kinetically. The SO₄-adsorption isotherm indicated that adsorption decreased with increased pH of the goethite suspension. The triple-layer model fit the experimental data well when adsorbed SO₄ was assumed to be located at the β plane and held via outer-sphere complexation. Pressure-jump relaxation was used to determine the kinetics of SO₄ adsorption/desorption on goethite. A single relaxation was observed due to adsorption/desorption processes. Based on the equilibrium and kinetic studies, we hypothesized and tested two reaction mechanisms for SO₄ retention. The first postulated mechanism (Reaction 1) assumed that SO₄ was adsorbed on a positively charged surface site through electrostatic attraction. This mechanism was rejected because of the poor conformity between the intrinsic equilibrium constant (K^int_eq) determined from the static studies and that found from the ratio of forward (k^int₁) and backward (k^int_-1) rate constants, i.e., K^int₁ = k^int₁/k^int_-1. It was found that the adsorption of SO₄ on goethite occurred simultaneously with the protonation of a neutral surface site, as assumed in the second hypothesized mechanism (Reaction 2). Based on this mechanism, a linear relationship between the reciprocal relaxation time (τ⁻¹) and various concentration terms was observed. From this relationship, k^int₂ and k^int_-2 were calculated and were 2.02 × 10⁸ mol⁻² L²s⁻¹ and 0.144 s⁻¹, respectively. The intrinsic equilibrium constant from kinetic measurements (K^int₂) was 10^9.14 mol⁻²L², which was similar to the intrinsic equilibrium constant from the equilibrium studies, 10^9.60.