Phototransformation of the Aqueous Fungicide Thiabendazole by Direct Photolysis and Heterogeneous Photocatalysis

Abstract

[Abstract] This study focuses on the degradation of the postharvest fungicide thiabendazole (TBZ), which is widely used in citrus production and veterinary medicine. Although TBZ is effective in controlling fungal infections, its residues have been detected in agricultural runoff, industrial wastewater, and surface and groundwater, demonstrating its resistance to conventional water treatment methods. Additionally, TBZ is highly toxic to aquatic organisms and has been classified as a possible carcinogen at high doses. To address this issue, advanced oxidation processes (AOPs), particularly heterogeneous photocatalysis, have emerged as promising solutions. Titanium dioxide (TiO2) is one of the most effective photocatalysts due to its chemical stability, low cost, and strong oxidizing ability when exposed to UV radiation. This study examines the phototransformation of TBZ using TiO2 P25, evaluating its degradation products and proposing a set of reaction pathways. The phototransformation of aqueous thiabendazole (TBZ), using commercial TiO2–P25 as a photocatalyst and UV irradiation (mainly 365 nm), is much faster than using UV irradiation (254 nm) alone. All kinetic runs were carried out at natural pH (ca. 6.1) and 298 K. The rate constant for photocatalyzed TBZ disappearance was ca. (4.8 ± 0.5)·10–3 s–1 (t1/2 ca. 3.5 min), with a mineralization of ca. 67% after 2 h, relative to the initial concentration of fungicide. Aqueous TBZ also underwent 254 nm induced phototransformation, with a rate constant of disappearance ca. (5.0 ± 0.1)·10–5 s–1 (t1/2 ca. 221 min), and just 3% of mineralization after 340 min. The quantum yield for direct phototransformation was Φ = 0.05, and the reactive species was identified as the singlet excited state of TBZ, as the concentration of dissolved O2 does not affect the phototransformation process. Up to 11 photoproducts were identified by HPLC-MS, the main one being benzimidazole. A suitable set of reaction pathways that may explain these products are proposed.