Degradation of disperse azo dyes from waters by solar photoelectro-Fenton

Highlights: → Reactive azo dyes are almost totally mineralized by solar photoelectro-Fenton. → The process yields high current efficiencies and low energy consumptions. → It is more efficient and inexpensive by decreasing current and increasing dye content. → Nitrate ions are the main inorganic ions released during the mineralization process. → The process is also effective for the remediation of textile dyeing solutions. - Abstract: Solutions of the azo dyes Disperse Red 1 (DR1) and Disperse Yellow 3 (DY3), commonly used in the Chilean textile industry, in 0.1 mol dm-3 Na2SO4 and 0.5 mmol dm-3 Fe2+ of pH 3.0 were comparatively degraded by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) using a 2.5 dm3 recirculation flow plant containing a BDD/air-diffusion cell coupled with a solar photoreactor. Organics were oxidized in EF with hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between electrogenerated H2O2 and added Fe2+. The oxidizing power of SPEF was enhanced by the additional production of hydroxyl radicals from the photolysis of Fe(III) hydrated species and the photodecomposition of Fe(III) complexes with intermediates by UV light of solar irradiation. Total decolorization, complete dye removal and almost overall mineralization for both dye solutions were only achieved using the most potent SPEF process, yielding higher current efficiencies and lower energy consumptions than EF. Final carboxylic acids like pyruvic, acetic, oxalic and oxamic were detected during the SPEF treatments. NO3- ion was released as inorganic ion. The use of a solution pH of 2.0-3.0 at 50 mA cm-2 was found preferable for SPEF. Synthetic textile dyeing solutions containing the dyes were treated under these conditions yielding lower decolorization rate, slower dye removal and smaller mineralization degree than only using 0.1 mol dm-3 Na2SO4 due to the parallel oxidation of organic dyeing components. However, lower energy consumptions were obtained by the destruction of more amounts of total organic carbon, indicating that SPEF is a useful and viable method for the remediation of textile industrial wastewaters with high contents of disperse azo dyes.