The kinetics of oxygen evolution reaction at platinum anodes in acid solutions with different pH's is examined, and possible reaction mechanisms are discussed. The activity of the reaction at initially prereduced electrodes brought to a constant potential, or kept at a constant current density, decreases with time. Only at the electrodes initially subjected for a given time either to a high potential, or to a high anodic current density, it is possible to observe in “run down,” or subsequent “run up” experiments a linear V‐logi relationship for over several decades of current density with a slope close to 115 mV. The positions, but not the slopes of Tafel lines, are affected by time or conditions of pretreatments of electrodes (various V or i ). The decrease in the activity is attributed to the increase in average thickness of anodically formed surface oxide films. Only when it is assured that the film thickness remains practically unaltered during the i‐V measurements, meaningful kinetic parameters for the oxygen evolution reaction are obtained. The reaction order with respect to H+ is then found to be negative and fractional according to the over‐all rate equationi=kc−1/2exp)(boldFnormalΔV2RTA model is suggested according to which a surface oxide film forms a barrier to charge transfer in series with the double‐layer barrier. The fractional reaction order is accounted for with this model. A chemical step that follows a charge transfer step is rate determining. It is suggested that oxygen atoms in the surface oxide films participate in the oxygen evolution reaction.

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BSTRACT: Mechanism and performances of arsenic(lll) [As(III)] and arsenic(V) [As(V)] sorption onto hydrated iron(lll) oxide (HFO)-coated materials were investigated at neutral pH where arsenic occurs in both molecular and ionic forms. Arsenic sorption by HFO-coated materials was proven to be a multistage process consisting of both macropore and intraparticle diffusion. Higher mass-transfer velocities were obtained for As(lII). which is attributed to the beneficial features of HFO. Equilibrium studies revealed the spontaneous and favorable nature of the arsenic sorption process. The maximum sorption capacity and the Gibbs free energy values indicated that HFO-coated materials exhibit more affinity towards As(IIl). The Langmuir and Freundlich isotherm models revealed both the chemical and physical nature of the sorption process, while the Dubinin-Radushkevich model indicated that physical sorption is a more dominant process with HFO-coated materials. Water Environ. Res., 83, 498 (2011).