Photolysis of ferric ions in the presence of sulfate or chloride ions: implications for the photo-Fenton process

The photo-Fenton process ($Fe^{2+}/Fe^{3+}$, $H_2O_2$, UV light) is one of the most efficient and advanced oxidation processes for the mineralization of the organic pollutants of industrial effluents and wastewater. The overall rate of the photo-Fenton process is controlled by the rate of the photolytic step that converts $Fe^{3+}$ back to $Fe^{2+}$. In this paper, the effect of sulfate or chloride ions on the net yield of $Fe^{2+}$ during the photolysis of $Fe^{3+}$ has been investigated in aqueous solution at pH 3.0 and 1.0 in the absence of hydrogen peroxide. A kinetic model based on the principal reactions that occur in the system fits the data for formation of $Fe^{2+}$ satisfactorily. Both experimental data and model prediction show that the availability of $Fe^{2+}$ produced by photolysis of $Fe^{3+}$ is inhibited much more in the presence of sulfate ion than in the presence of chloride ion as a function of the irradiation time at pH 3.0.