Kinetic Approach by Photocurrent Measurements to the Photoelectrocatalytic Oxidation of an Anionic Surfactant Using an S,N-TiO2/Ti Electrode: Distinguishing Between Direct and Indirect Mechanisms

The photoelectrocatalytic oxidation of an anionic surfactant (internal olefin sulfonate C20-C24, IOS) in synthetic oilfield wastewater was investigated by photocurrent measurements using a sulfur and nitrogen co-doped titanium dioxide electrode (S,N-TiO₂/Ti). For the electrode preparation, S,N-TiO₂ films were supported on Ti expanded meshes by sol–gel dip-coating, followed by thermal treatment at 400 °C. Photocurrent measurements were performed by linear sweep voltammetry (LSV) under UV–Vis irradiation using different IOS concentrations (20–200 ppm). The photocurrent values obtained at 0.5 V vs Ag/AgCl (i_ph) for each IOS concentration (C_IOS) were used to plot the 1/i_ph vs 1/C_IOS graph, in which two trends were identified. For high IOS concentrations (70–200 ppm), the values fitted well to the unimolecular Langmuir–Hinshelwood (LH1) kinetic model (R²?=?0.973), which can be associated with the direct oxidation mechanism. For low IOS concentrations (20–70 ppm), the values fitted better to a linear combination of both unimolecular and bimolecular Langmuir–Hinshelwood (LH1 and LH2) kinetic models (R²?=?0.854), which can be associated with direct and indirect oxidation mechanisms, respectively. These results suggest that at high IOS concentrations the IOS adsorption plays the main role on the photoelectrocatalytic oxidation, while at low IOS concentrations both IOS adsorption and H₂O adsorption (surface hydrophilicity) play important roles on the photoelectrocatalytic oxidation.