Research study of the operational parameters affecting the different Fenton processes to reduce the presence of dyes in textile effluents

Abstract

Water pollution is an environmental problem that has increased over the years, due to an increase in water consumption by the population and industry. The textile industry is the second most water-consuming manufacturing, and therefore generates a considerable amount of wastewater, which needs to be accurately treated for the re-use. These wastewaters are characterised by having highly toxic organic compounds and colouring agents, which are part of the group of emerging pollutants (EPs). Emerging pollutants (EPs) are compounds present in the environment that have high toxicity and persistence. Besides, these compounds begin to be regulated by law, and there are concerns about the possible consequences they may cause. The elimination of most of these emerging contaminants cannot be carried out by conventional treatments due mainly to their low biodegradability and high stability. Advanced oxidation processes (AOPs) are alternative treatments capable of eliminating these poorly biodegradable compounds. These techniques, based on the generation of high oxidizing power species such as hydroxyl radicals (·OH), can oxidize chemical compounds and completely mineralize the organic contaminants present in wastewater. The different Fenton processes are also part of the group of advanced oxidation processes and are very promising technologies for the treatment of textile effluents. The purpose of this work is to investigate the different operational parameters that affect the efficiency of Fenton processes, for the abatement of two representative dyes, indigo carmine and methyl orange. It is performed a bibliographical revision of several scientific papers, which focus on explaining and studying some of the factors that influence on the degradation process of these dyes. The following techniques have been considered: classical Fenton, Photo-Fenton, Photo-Fenton-Like, Electro-Fenton and Photo-Electro-Fenton. The use of a lower dye concentration, a Boron-Doped Diamond (BDD) anode in the Electro-Fenton process, the incorporation of ultraviolet radiation or the need to acidify the solution, up to a pH value around 3, are some essential factors that allow better abatement of the dyes, according to the consulted literature. Besides, this document also shows an experimental part, in which is performed the discolouration of an indigo carmine solution though the Photo-Fenton process at circumneutral pH, using Diethylenetriaminepentaacetic acid (DTPA) as the chelating agent to avoid iron precipitates, thus making the process more applicable on an industrial scale. The process to evaluate occurs in a reactor using different artificial light sources (black light blue, BLB and light-emitting diode, LED), with different BLB lamp powers (8 W and 24 W) and with different initial concentrations of hydrogen peroxide (H2O2, 200 ppm and 100 ppm). The best results from the experimental part are obtained using BLB lamps with a 24 W power and an initial oxidant concentration of 200 ppm. A 98.1% of discolouration was achieved in 100 minutes of reaction, using concentrations of 50 ppm of indigo carmine dye and 5 ppm of Fe2+. Besides, the application of DTPA as a chelating agent has allowed the catalyst (Fe2+) to remain soluble