Advanced oxidation processes for the removal of dyes from synthetic industrial wastewaters

Recent studies have demonstrated the efficiency of hydrodynamic cavitation (HC) for treating industrial wastewaters polluted by organic substances. HC provides the production of radicals OH, due to the passage of the aqueous solution through a constriction (an orifice plate or Venturi tube). The radicals react with organic pollutants to form intermediate compounds more readily degraded by the biological processes, or the degradation may lead to complete mineralization. In the present work, the HC process is applied to treat methylene blue (MB) solutions. The core of the experimental apparatus consists of a Venturi tube having an orifice of 2 mm. The effect of inlet pressure on Venturi, solution pH, and initial dye concentration is investigated on MB decolorization yield. The maximum yield is obtained at 0.55 MPa, pH = 2, and an initial dye concentration of MB of 5 ppm. The results have been compared with those obtained for another azo-dye, the methyl orange, in the same experimental conditions, showing comparable results with using HC alone, with degradation efficiencies lower than 30%; the results are unsatisfactory if the HC is used as a stand-alone technique, but can be sufficient if HC is intended as a pre-treatment step prior to a biological process.