Oxidation of tetracycline and oxytetracycline for the photo-Fenton process: Their transformation products and toxicity assessment

Advanced oxidation processes have gained significant attention for treating tetracycline (TC) and oxytetracycline (OTC), however, their oxidation using the photo-Fenton process has not been sufficiently studied. Although degradations of TC and OTC were enhanced by increasing H₂O₂ and Fe²⁺ within the ranges investigated (H₂O₂ = 20–50 mg/L and Fe = 1–10 mg/L) under UV irradiation, further experiments for the photo-Fenton process were conducted with 20 mg/L of H₂O₂ and 5 mg/L of Fe²⁺ to balance efficiency and cost. The photo-Fenton process (UV/H₂O₂/Fe²⁺) was shown to be more effective to remove TC and OTC than H₂O₂, ultraviolet (UV), and UV/H₂O₂ at the same doses of oxidants. Inorganic anions and cations were shown to inhibit the degradation of TC and OTC during the photo-Fenton process, in the following order: HPO₄^2- > HCO₃⁻ ≫ SO₄^2- > Cl⁻ and Cu²⁺ ≫ Ca²⁺ > Na⁺. The TC and OTC degradation are generally improved by increasing pH, which is opposite to the k_pCBA,obs values, caused by increasing the deprotonation degree of TC and OTC. Four and nine transformation products of TC and OTC, respectively, were detected over the treatment period. Among the transformation products, m/z 443.14 (C₂₂H₂₂N₂O₈) formed during TC degradation, and m/z 433.16 (C₂₀H₂₀N₂O₉) and m/z 415.15 (C₂₀H₁₈N₂O₈) formed during OTC degradation, were reported for the first time. Vibrio fischeri toxicity assessment indicated that the inhibition ratio was decreased with a decreasing TC concentration, while, OTC transformation lead to higher toxicity. The product (m/z 477.15b) was determined to be the compound causing toxicity during degradation of OTC by using the quantitative structure activity relationship (QSAR). This toxic transformation product caused higher inhibition ratios than its parental compound (OTC), but its further oxidization resulted in decreasing the inhibition ratios.