Kinetics and Mechanisms of Sulfate Adsorption/Desorption on Goethite Using Pressure‐Jump Relaxation

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Abstract
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 (Kinteq) determined from the static studies and that found from the ratio of forward (kint1) and backward (kint‐1) rate constants, i.e., Kint1 = kint1/kint‐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, kint2 and kint‐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 (Kint2) was 109.14 mol−2L2, which was similar to the intrinsic equilibrium constant from the equilibrium studies, 109.60.

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