Removal of methylene blue from aqueous solutions using an Fe2+ catalyst and in-situ H2O2 generated at gas diffusion cathodes

Textile industries generate large volumes of wastewater containing organic dyes, which may be hazardous to the environment and human health. In this work, the oxidation of 100 mg dm-3 of methylene blue (MB) in aqueous solutions was studied using H2O2 (formed in-situ) at gas diffusion electrodes (GDEs) in the presence of 1 × 10-3 mol dm-3 Fe2+ catalyst, to create hydroxyl radicals to facilitate Fenton oxidation. The influence of applied potential (-0.5 to -1.3 V vs. Ag/AgCl) at different oxygen flow rates to the GDE (0.15 – 0.45 cm3 min-1) and different counter electrode materials (Pt mesh, Ni mesh, RVC) was investigated. MB was completely degraded to a residual concentration below the detection limit of 0.5 mg dm-3. The rate and degree of mineralisation were significantly influenced by the applied potential. A maximum mineralisation of 88.2% was achieved at -1.0 V vs. Ag/AgCl. The oxygen flow rate to the GDE did not influence the degradation of MB under the experimental conditions. The counter electrode material affected the degree of mineralisation in the order Pt mesh > reticulated vitreous carbon (RVC) > Ni mesh. The apparent first order rate decay constant for MB degradation and MB mineralisation was calculated to have a maximum value of 0.0182 min-1 for MB mineralisation at a potential of -1.0 V vs. Ag/AgCl