Tratamento de águas contaminadas com corantes azóicos pela aplicação de processos oxidativos avançados fotoquímicos, eletroquímicos e fotoeletroquímicos utilizando radiação UVA e solar

Abstract

Advanced oxidation processes (AOPs) are promising technologies for environmental remediation. These technologies are based on the high oxidation power of hydroxyl radicals (•OH), which have a high reactivity to oxidize organic compounds present in liquid effluents. In this work, the application of different AOPs to treat synthetic and real solutions containing azo dyes has been studied. These compounds present a serious socio-environmental risk since they have high toxicity and are resistant to conventional treatment methods. The obtained results were presented in four distinct chapters. In chapter 1, two valuable natural resources of Brazil were used, namely, the sun in the Northeast region and niobium, revealing that solar photocatalysis whith niobium oxide (Nb2O5) is an effective and economical method for discoloring dyes at low concentrations. The photocatalytic efficiency of Nb2O5 was evaluated varying the concentrations of dye, H2O2, Nb2O5 and pH. The best operating conditions achieved were 1 g L-1 of Nb2O5, 5 mg L-1 of dye and 0.20 M of H2O2 and pH 5. At the end of five treatment cycles, Nb2O5 photocatalyst efficiency remained constant. In the case of chapters 2, 3 and 4, electrochemical and photoelectrochemical treatments using UVA and solar radiation were used. In chapter 2, a solution containing the dye diazo Pounceau SS was treated using a pre-pilot flow plant equipped with a boron doped diamond (BDD) or platinum (Pt) anode and air diffusion cathode, connected to a photoreactor containing a 160 W UVA lamp. Anodic oxidation with electrogenerated H2O2 (AO-H2O2) was more efficient with BDD, due to the greater oxidation capacity of heterogeneous •OH radicals formed on its surface. In electro-Fenton (EF) and photo electro-Fenton (PEF), the •OH radicals produced by Fenton’s reaction were more reactive. The PEF had a better performance due to the additional photolysis of intermediates and photodecarboxylation of Fe (III) complexes with UVA light. The influence of light minimized the effect of the anode in the process, which is relevant in AO-H2O2 and EF processes. In chapter 3 was evaluated the influence of water hardness on the color treatment of eriochrome black T by means of the EF process. The dye molecule forms complexes with calcium and magnesium. These complexes being stable make difficult the electrophilic attack of •OH radicals. Thus, water hardness should be an highlihted parameter in the EF process. In chapter 4, it was taken into account that daily activities, as washing clothes, generate dye effluents that need to be treated. A viable alternative is to consider the wastewater treatment at a domestic scale. Thus, AOPs were applied to an actual laundry washing machine effluent, demonstrating high efficiency performance and that they could be used as decentralized small scale treatment devices. Solar PEF showed to be a suitable process for complete removal of color and organic matter after 240 min of treatment applying a current density of 66.6 mA cm-2. AOPs studied in this work proved to be satisfactory and sustainable technologies for the treatment of liquid effluents.