Degradação de pesticidas por processos oxidativos avançados

Abstract

Due to the low efficiency in the degradation of emerging contaminants by conventional treatments, Advanced Oxidative Processes (AOPs) have been used to efficiently degrade them in environmental matrices. In this work, different processes were evaluated for the degradation of pesticides in sewage treatment plants (STP) and in distilled water (DW) at a pH close to neutral (pH 6-7). In the first step, the photo-Fenton process modified with ethylenediamine-N,N′-disuccinic acid (EDDS) was evaluated for degradation of a mixture of pesticides (5 µmol L–1 of each) ametrine (AMT), atrazine (ATZ), imidacloprid (IMD) and tebuthiuron (TBH). Initially, the operational parameters (concentration of Fe3+-EDDS, molar ratio Fe:EDDS, mode of addition of EDDS and radiation source) were evaluated, and removals of 80–98% of target pesticides were obtained using 300 µmol L–1 of H2O2 and 30 µmol L–1 of Fe3+-EDDS in a molar ratio of 1:2 in DW, being more efficient than the classic photo-Fenton process (30–84% removal). However, due to the influence of inorganic ions and natural organic matter from the STP effluent, it was necessary to increase the reagent concentration six times (180 µmol L–1 of Fe3+-EDDS and 1800 µmol L–1 of H2O2) to reach the same proportion as in DW. Acute toxicity for Vibrio fischeri was evaluated during treatment in DW, and, despite the formation of more toxic intermediates, these products were degraded throughout the treatment, since there was no bacteria inhibition at the end of the process. In STP it was observed a reduction in toxicity to Drosophila melanogaster flies and antibacterial activity for Escherichia coli. Subsequently, the degradation of TBH alone (100 µg L–1 in DW and 500 µg L–1 in STP) was evaluated, due to its higher recalcitrance. In this step, UV-C radiation was evaluated with different peroxides (H2O2, S2O82– and HSO5–), reaching limit of quantification (12 µg L–1) in both matrices (88% degradation). Fourteen transformation products (TP) were identified, one of them reported for the first time in the literature, and, considering the radical species involved (HO• and SO4•–), two degradation mechanisms were proposed: one by abstraction and the other by electron transfer. Using Eisenia andrei earthworms, it was observed that at the end of the UV-C/peroxides processes there was no formation of more toxic by-products. Therefore, the proposed treatments are interesting alternatives for the degradation of contaminants of emerging concern and effective for the reduction of toxicity in DW and STP effluents.