Pulsed light for a cleaner dyeing industry: Azo dye degradation by an advanced oxidation process driven by pulsed light

Abstract

Water pollution by wastewater containing dyes is an environmental issue that can be mitigated by the use of advanced oxidation processes (AOP). Pulsed light (PL) is an emerging food processing technology that uses eco-friendly lamps and can potentially be adapted as light source of an UV-based AOP. In the present work, a PL/H2O2 process was tested for the decolourization of two azo dyes, and a pulsed light/H2O2/ferrioxalate process was tested for the decolourization of one azo dye. The efficiency of the PL/H2O2 process in a batch reactor under different parametric values: dye concentration, pH, H2O2 and salt doses was followed by spectrophotometry and fitted to first-order kinetics; and several degradation products were detected. In the PL/H2O2 process, decolourization rates increased at low dye concentrations and high H2O2 doses, were pH-dependent and were inhibited by the addition of NaCl, Na2SO3 or Na2CO3. More than 50 % decolouration was achieved with the PL/H2O2 process for both dyes after applying 54 J/cm2 (25 light pulses). The PL/H2O2/ferrioxalate process achieved > 95 % decolouration for Methyl orange when applying 21 J/cm2 (10 pulses); that level of energy can be supplied by PL commercial systems in nine and four seconds respectively. No known toxic degradation products were detected. Results show that a PL/H2O2 process has potential to be used for the efficient degradation of dyes from wastewater. Furthermore, the efficacy of this process can be improved by the use of ferrioxalate. PL technology could become an alternative light source to contribute to decrease the environmental impact of wastewater produced by the dyeing industry.We are very thankful for all the suggestions. We have never had so many suggestions for improving a manuscript, and it has taken much time to fulfill all the requests, including new determinations. However, we consider that the requests have been very useful for improving our manuscript.Besides attending all remarks, we inform that we have introduced two additional changes.The first one is that we have deleted all information related to Direct red 83:1 dye. This decision was taken because we were unable to detect intermediaries of the degradation of this dye in spite of we sub-contracted a top analytical laboratory. Therefore, we believe that we should not report the full data from three dyes (Congo red, Direct red 83:1 and Methyl orange) but reporting intermediaries for only two. This decision also took into consideration that the validity of the study and its conclusions also remain valid in spite of excluding the third day from our manuscript. Furthermore, if data on Direct red 83:1 would have left, it would have made the manuscript excessively long, since new data and discussion was added after reviewer's comments.The second change is that we have included a new co-author. She is the leader of the laboratory that was sub-contracted for the study of degradation products. We believe that her effort and input has gone beyond what we have paid, and that she has made a signifi...

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