Estudo de fotodegradação de corantes xantênicos utilizando os sistemas UV/TiO2 e UV/S2O8

Abstract

The presence of dyes in water reservoirs not only gives an atypical colour to water, but also represents socio-environmental problems. In this context, advanced oxidative processes (AOPs) emerge as methodologies that aim to degrade organic pollutants. Among the AOPs, photodegradation has been used to produce radical oxidizing species, such as hydroxyl. The objective of this study is to compare the photodegradation methods of dyes belonging to the group of xanthenes, with different sources of oxidizing radicals. For this study, the dyes Rhodamine B, Yellowish Eosin and Sodium Fluorescein were subjected to photochemical reactions in the presence of TiO2 and persulfate (K2S2O8, called PS for simplification effect). The influence of UV radiation and the presence of persulfate in control systems were also evaluated. The experiments were followed by UV-Vis spectrophotometry, Total Organic Carbon (TOC) and fluorescence spectroscopy. Theoretical calculations using Fukui condensed functions were used for comparison with experimental results. It was verified through control systems (experiments without light) that the combination of TiO2 and PS with UV radiation contributes to the formation of oxidant radicals SO4● − and HO●. In pH values of 3.0 and 11.0 a higher percentage of photodegradation of dyes was observed, and therefore these conditions were adopted in other studies. When using the system with UV/PS at pH 11.0 photodegradation of 70.6%, 94.5% and 97.4% for RhB, EY and Fl, respectively, was observed in 180 minutes. On the other hand, when using UV/TiO2 at pH 11.0, the percentage of degradation was greater than 99% in 90 minutes for the three dyes studied. The reactional kinetics of the UV/PS staining system follows the pseudo-first order model and it was found that, among the dyes studied, Fluorescein is the most dependent on the presence of radicals. The TOC analyses showed a mineralization for RhB, EY and Fl in 82.32%, 85.21% and 87.54% when using persulfate at pH 3.0 and percentages of 61.65%, 68.17% and 71.01% were obtained when using TiO2 at pH 11.0, respectively. A study of the participation of radicals in degradation by persulfate, using ethanol and tert-butanol as radical sequestrants, suggests that the reaction occurs by two routes, radical (SO4● − and HO●) and not radical. Studies of inhibition of radicals in TiO2 degradation at pH 3.0 and 11.0, indicate a drop of 41.1 and 93.3 % suggesting the action of the hydroxyl radical. Theoretical calculations using Fukui condensed functions were associated with the spectral variation of dye degradation and gave indications about the preferred degradation pathways.