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Kinetics and Mechanisms of Sulfate Adsorption/Desorption on Goethite Using Pressure‐Jump Relaxation
Author(s) -
Zhang Peng Chu,
Sparks Donald L.
Publication year - 1990
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1990.03615995005400050011x
Subject(s) - adsorption , chemistry , goethite , desorption , relaxation (psychology) , kinetics , aqueous solution , equilibrium constant , reaction rate constant , temperature jump , analytical chemistry (journal) , chromatography , psychology , social psychology , physics , quantum mechanics
Sulfate adsorption/desorption on goethite (FeOOH) in aqueous suspension was investigated thermodynamically and kinetically. The SO 4 ‐adsorption isotherm indicated that adsorption decreased with increased pH of the goethite suspension. The triple‐layer model fit the experimental data well when adsorbed SO 4 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 SO 4 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 SO 4 retention. The first postulated mechanism (Reaction 1) assumed that SO 4 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 int 1 ) and backward ( k int ‐1 ) rate constants, i.e., K int 1 = k int 1 / k int ‐1 . It was found that the adsorption of SO 4 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 int 2 and k int ‐2 were calculated and were 2.02 × 10 8 mol −2 L 2 s −1 and 0.144 s −1 , respectively. The intrinsic equilibrium constant from kinetic measurements ( K int 2 ) was 10 9.14 mol −2 L 2 , which was similar to the intrinsic equilibrium constant from the equilibrium studies, 10 9.60 .