Potential of organic iron complexes and peroxide sources as alternative degradation of the mixture of benzophenone-3, fipronil and propylparaben in tertiary wastewater under solar radiation

Abstract

This Ph.D. Thesis focuses on the use of Fe-citrate (FeCit) and Fe-nitrilotriacetic acid (FeNTA) complexes and different peroxide sources (hydrogen peroxide (H2O2), persulfate (S2O82–) and monopersulfate (HSO5–)) under solar radiation for the degradation of the mixture of benzophenone-3 (BP-3), fipronil (FIP) and propylparaben (PPB) (100 µg L–1 each, added in tertiary sanitary effluent at pH 7.4). In Chapter I, the theme was contextualised, followed by a conceptual approach to the key elements of this research. In Chapter II, a state-of-the-art review of advanced oxidation processes for the degradation of these compounds was carried out and the literature points to a satisfactory degradation, with formation of less toxic transformation products for BP-3 and PPB. However, the low degradation rates achieved in real matrices impose a challenge for full-scale application. Thus, in chapter III the performance of these complexes in the presence of H2O2 on degradation of the mixture of the three micro-pollutants was evaluated. Degradation efficiencies were significantly improved by increasing the iron concentration (Fe/L (1:1)) from 12.5 to 100 µmol L–1 for FeNTA, whereas degradation achieved with FeCit was limited to 30% due to the lower stability of the FeCit complex, and a higher Fe/Cit molar ratio (1:5) was required to maximise degradation efficiency. For the best Fe/L molar ratios, higher degradation rates were achieved using 5.9 mmol L–1 of H2O2 for FeNTA and 2.9 mmol L–1 of H2O2 for FeCit and a lower concentration of the oxidant was required in the FeCit system due to the in situ generation of H2O2 at pH 7.4. In chapter IV, the performance of both complexes in the presence of S2O82– and HSO5– under the best experimental conditions determined for H2O2 was evaluated. Degradation rates with S2O82– were limited to 30% for FeNTA and 55% for FeCit while the addition of 5.9 mmol L–1 of HSO5– resulted in acidic solutions and rates above 96%. Higher performance of S2O82– was obtained at acidic pH and after combined removal of chloride, bicarbonate and sulphate ions. The low degradation rates with S2O82– at pH 7.4 was also influenced by the structure of the target compounds. The acute toxicity to D. Melanogaster reduced significantly after treatment for both iron complexes with H2O2, indicating the formation of low-toxicity by-products. Although FeCit provided higher kinetics, its use inserts higher organic loading, resulting in higher cost-effectiveness (20 US$ m–3 versus 13 US$ m–3 para FeNTA) and emphasizing the importance of evaluation and optimization of the operational parameters for each ligand and peroxide source.