Naturally occurring biopterin as a photosensitizer for enhanced photodegradation of sulfonamides in aquatic environments.

The natural photosensitizer Biopterin (Bip) can significantly accelerate the photodegradation of sulfamethazine (SMZ). Under simulated sunlight, 5 μM Bip achieved a 3-h degradation rate of 69.06 ± 2.50% for SMZ, while direct photolysis was less than 3%. The reaction rate increases linearly with the concentration of Bip, with the optimal pH ≈ 8. HCO3- and Cl- enhanced SMZ degradation, likely through the generation of reactive secondary species and ion-mediated processes. Laser flash photolysis and quenching experiments confirmed that the triplet excited state (3Bip∗) of Bip is the main active intermediate, while the singlet oxygen and hydroxyl radicals have relatively minor effects. DFT calculations show that the optical band gap of Bip is 3.02 eV, and the electron transfer ΔG from 3Bip∗ to SMZ is -0.87 eV, which initiates the reduction cleavage and desulfonylation of the S-N bond of sulfonamides. The main products are hydroxylated and desulfonylated derivatives, with changes in polarity and molecular weight. The ecotoxicity assessment indicated that the toxicity of the product was significantly enhanced: the 72-h EC50 of raphidocelis subcapitata decreased by 4.8 times, the 96-h mortality rate of Danio rerio embryos increased by 3.2 times, and the fixation rate of Daphnia magna increased by 2.6 times. This study indicates that Bip is an unknown environmental factor. Although its photochemical transformation can degrade antibiotics, it generates more toxic by-products, suggesting that the toxicity of photoproducts should be included in the supervision of drug pollution in water bodies.