Surface oxidation pre-treatment on activated carbon: Effect on its cathode performance in electro-Fenton processes

In this work, activated carbon (AC) of vegetable origin was exposed to diluted HNO3 solutions for different time periods in order to promote varying degrees of oxidation of the carbon surface particles. In this way, activated carbon samples immersed in 10 % HNO3 solutions for 0, 6, 12, 24 and 48 h, resulted in well-defined proportions of carbonyl, lactone, carboxylic and phenolic groups on the surface of the carbonaceous material that, as expected, showed characteristic adsorption properties for anionic as well as for cationic dye molecules (methyl orange and methylene blue, respectively) under neutral pH conditions. The AC material in a packed bed set-up as a gas diffusion electrode (GDE), also showed that oxidative treatment favors the oxygen reduction reaction due to the formation of lactones. Experiments using these materials in electro-Fenton processes revealed that, as opposed to what happens in the bulk of the solution when a solvated anionic dye molecule is employed, the adsorption of methylene blue in the surface of AC results in a Langmuir-Hinshelwood kinetics behavior (characterized by kinetic constant (K-1) of 0.271 mLs(-1)mol(-1) and a theoretical Delta G of 0.428 kcal /g AC) whose decolorization rate becomes maximized when a treatment time of 6 h is employed.