Degradation mechanism of fipronil and its transformation products, matrix effects and toxicity during the solar/photo-Fenton process using ferric citrate complex

Abstract This study presents the degradation of fipronil in sewage treatment plant (STP) effluent by photo-Fenton at near neutral pH (pH 6.0) using Fe3+/Citrate complex. 83% of fipronil degradation was reached using a molar iron/citrate ratio of 1:3 (192 μmol L−1 of Fe3+/576 μmol L−1 of citrate). Photo-Fenton reduced the toxicity of treated solutions as according to the survival of Drosophila melanogaster exposed to non-treated and treated samples. Control experiments performed in distilled water using 32 μmol L−1 of Fe3+/96 μmol L−1 of citrate achieved 98% of fipronil degradation within 100 kJ m−2 (UV-A radiation, k = 30 × 10−3 kJ−1 m2 and t1/2 = 23 kJ m−2), thus indicating that fipronil degradation is impaired by natural organic matter and inorganic ions present in STP effluent. Degradation was faster under solar radiation, as the same efficiency (98%) was obtained after 75 kJ m−2 (k = 63 × 10−3 kJ−1 m2 and t1/2 = 11 kJ m−2). In addition, pathways of fipronil degradation using Fe3+/Citrate under solar and UV-A radiation were investigated and transformation products proposed. Results revealed that the HO• attack occurred preferentially in the pyrazole ring. Eight transformation products were identified by UHPLC-Q-TOF-MS and four are unprecedented in the literature. Control experiments in distilled water demonstrated that toxicity reduction is related to fipronil degradation and that transformation products are less toxic than fipronil. Furthermore, toxicity of STP fortified with fipronil was reduced after photo-Fenton. These results demonstrate the feasibility of applying this process using Fe3+/Citrate complex for fipronil degradation in a real matrix.