The efficiency of Fenton, Fenton/MW and UV/oxidant processes in the treatment of a mixture of higher concentrations of azo dyes

•Mixing of dyes causes rapid decolorization and slows mineralization vs. single dyes.•MW irradiation and hydroxylamine increased the efficiency of the Fenton reaction.•Mineralization by Fe2 + /SPS is more effective than Fe2 + /HP has been proved wrong.•Sodium Persulfate does not work at higher concentration of the target molecule.•Different levels of toxicity are shown by V. fischeri and E. coli. The concentration of dyes in the degradation process, as found in the literature, higher than 10 – 30 ppm is rarely found in the literature. The present study attempts to establish the degradation efficiency of a mixture of two azo dyes, Acid Blue 29 and Ponceau Xylidine at higher concentrations by Fenton, Fenton-type, photolysis and microwave assisted Fenton processes, employing hydrogen peroxide (HP) and sodium persulphate (SPS) as oxidants. It was observed that mixing of dyes enhances decolorisation and retards chemical oxygen demand (COD) and total organic carbon (TOC) removal in Fenton process. Nevertheless, COD removal efficiency (CODeff) was higher at pH 3 than at pH 5.9, the respective values being 72.8 and 61.2% in 300 min. Although, the COD removal in Fe2+/HP system was faster in the early stages than in UV/HP system, the later shows higher CODeff than the former (96.6 and 72.8%, respectively, in 300 min). This result is in good agreement with the consumption pattern of HP in both systems. Although contradicting in nature, our finding, that the inhibition of degradation by persulphate, is remarkable. Although, microwave (MW) irradiation accelerated Fenton process, the overall COD removal was not increased. The effect of inorganic ions and hydroxylamine on the Fenton process has been reported in this study. Toxicity assessment by both colony forming units (CFUs) and light loss method shows that the detoxification efficiency of different processes are in the order UV/HP>Fe2+ /HP. [Display omitted] (a) Acceleration of COD removal by Hydroxyl radical based AOP and inhibition of COD by persulfate based AOP; (b) different level of toxicity for a given sample as shown by E. coli CFU and V. fischeri light inhibition assay; (c) decolorization efficiency of different system