Evidence of cathodic peroxydisulfate activation via electrochemical reduction at Fe(II) sites of magnetite-decorated porous carbon: Application to dye degradation in water

Peroxydisulfate (PDS, S2O82−)-based advanced oxidation processes have been developed as an alternative to those based on OH, as PDS activation yields a much more stable radical like SO4− that can maintain the oxidation ability of water treatment systems for longer time. Here, the electrochemical PDS activation has been investigated using reticulated vitreous carbon (RVC) substrate modified with Fe3O4 nanoparticles (NPs) as cathode. The NPs were exhaustively characterized by different surface analysis techniques (TEM, SEM) and Mössbauer spectroscopy. Cyclic voltammetry and linear sweep voltammetry with a rotating disk electrode allowed concluding that the main electrocatalytic role in the cathodic PDS activation to SO4− corresponded to the Fe(II) active sites continuously promoted upon cathodic polarization. These sites were less catalytic for O2 reduction reaction, although it was still feasible with n = 2.7 electrons as determined from Koutecky-Levich analysis. Both cathodic reactions followed an inner-sphere reaction mechanism. The Fe3O4-modified RVC cathodes were employed to electrolyze Methylene Blue aqueous solutions at pH 3.5, employing different current values and PDS concentrations. Dissolved O2 was purged to impede the competitive cathodic H2O2 production and Fenton’s reaction. The occurrence of dye adsorption/electrosorption on the cathode reduced the mass transport limitations, enhancing the reaction between SO4− and organic molecules. The best operation conditions to reach total and fast color removal at 18 min were 2 mM PDS and 10 mA, yielding > 80% TOC abatement at 45 min. Reproducible degradation profiles were found after 5 runs, thereby ensuring the stability of the Fe3O4-modified RVC, with no iron sludge production.