Ferrioxalate complexes as strategy to drive a photo-FENTON reaction at mild pH conditions: A case study on levofloxacin oxidation

Abstract

Abstract The present study is about the use of ferrioxalate photoactive complexes as a strategy to drive a photo-Fenton reaction at mild pH conditions, applied to the oxidation of antibiotic levofloxacin (LEV) in pure water and spiked in urban wastewater after secondary treatment. The oxidation ability of the solar photo-Fenton mediated by ferrioxalate complexes (SPFF) was evaluated at different pH values (3.0 to 6.0) for an iron/oxalate molar ratio of 1:3 and using low iron contents (1.0 and 2.0 mg Fe 3+ L −1 ). Additionally, the effect of LEV concentration (2–20 mg L −1 ), iron:oxalate molar ratio (1:3, 1:6 and 1:9), temperature (15–45 °C), UVA irradiance (27.8-59.9 W UV m −2 ), presence of inorganic ions (Cl − , SO 4 2− , HCO 3 − , NH 4 + ), radical scavengers (sodium azide, humic acid and D-mannitol), and others organic ligands, namely citrate and malate, on the SPFF method at pH 5.0 was also assessed. Although the conventional solar photo-Fenton (CSPF) reaction showed similar results to SPFF for pH values ≤ 4, an almost 3.3-fold and 5.7-fold increase in the LEV degradation rate at pH 5.0 and 6.0, respectively, were observed for SPFF when compared to CSPF. This is mainly attributed to the higher photoactivity and solubility of ferrioxalate complexes when compared with ferric ion-water complexes. On the other hand, ferrioxalate complexes exhibited higher photoactivity on LEV removal than ferricitrate and ferrimalate. The presence of phosphate ions in the urban wastewater affected negatively the reaction rate, mainly due to the precipitation of iron as FePO 4(s) , hence a H 2 O 2 /UVC process at different conditions was also explored as an alternative. A pilot-scale assay under natural solar irradiation showed similar results to the ones obtained in the lab-scale photoreactor. Oxalic, formic, citric, and tartronic acids were identified as residual acids in SPFF at pilot scale.