Electromigration of protons and zero valent iron oxidation: A physico-chemical insight to model the kinetics of fenton-like process

To describe correctly the kinetics of Fenton-like process with zero valent iron, it is imperative to determine the rate of Fe2+ formation by ZVI oxidation at fairly acidic pH. With this purpose, experiments of ZVI oxidation were carried out in a well-stirred reactor in which air or N-2 was bubbled through an aqueous solution at pH 2.5, 3 and 4 and 25 ?degrees C for 10800 s. To mimic the Fenton-like process, the experiments were repeated at pH 3 and two different initial concentrations of H2O2. The results without H2O2 revealed that the rate-limiting step of Fe2+ formation is the electromigration of protons through an iron oxide barrier layer of negligible thickness. The solution of the Nernst-Planck equation was able to describe the flux of protons involved in the reduction half reaction of ZVI oxidation. From the rate of protons consumption and the stoichiometry of the overall redox reaction, a rate expression for Fe2+ formation was obtained. A novel detailed kinetic model for the Fenton-like process with ZVI involving 20 species and 55 reactions was suggested. The only tuned parameters were the effective diffusivity of protons times the electric potential gradient, and the rate constant of precipitation of iron ion species. The results of species concentrations in the experiments with H2O2, and a large set of independent data of concentration of major species in reactors where powder iron and steel plates were immersed in aqueous acidic solution with and without H2O2, support both the rate equation for the Fe2+ formation and the Fenton-like kinetic model.

Automated summary

The study experimentally determined the rate expression for Fe2+ formation in the Fenton-like process with zero valent iron, and proposed a detailed kinetic model, which was supported by experimental results and independent data.