Mechanism of Lead Ion Adsorption at the Goethite—Water Interface

The pressure—jump relaxation technique has been used to determine the mechanism of the reaction of lead ion with the goethite/solution interface. The kinetic study required three types of information to unambiguously specify the detailed reaction mechanism: (1) overall equilibrium, (2) reaction stoichiometry, and (3) reaction rate data. Electrical double layer models must be used in both equilibrium and time-dependent relationships for reactions occurring at charged interfaces. Based on the kinetic results of this study, a bimolecular adsorption/desorption reaction has been postulated for Pb2+ adsorption/desorption at the goethite/solution interface. The desorption step associated with breaking the bond of the inner-sphere lead hydroxyl surface complex is the ratelimiting step. Dependence of the rate constants with pressure-jump magnitude is consistent with adsorption/desorption from sites with a distribution of bond energies.