Environmental photochemical fate and UVC degradation of sodium levothyroxine in aqueous medium

Abstract

The synthetic hormone sodium levothyroxine (LTX) is one of the most prescribed drugs in the world and the most effective in hypothyroidism treatment. The presence of LTX in the environment has become a matter of major concern due to the widespread use of this hormone and by the fact that it is only partially removed in conventional water and sewage treatment plants. However, information regarding the photochemical fate of this hormone in environmental or engineered systems is scarce in the literature. In this work, the sunlight-driven direct and indirect LTX degradation was investigated by determining the photolysis quantum yield, Φ_LTX = 3.80 (± 0.02) × 10⁻⁵, as well as the second-order kinetic constants of the reactions with hydroxyl radicals, k_LTX,•OH = 1.50 (± 0.01) × 10¹⁰ L mol⁻¹ s⁻¹ and singlet oxygen, k_LTX,1O2 = 1.47 (± 0.66) × 10⁸ L mol⁻¹ s⁻¹. Mathematical simulations indicate that LTX photodegradation is favored in shallow, nitrite-rich, and dissolved organic matter (DOM)-poor environments, with LTX half-life times varying from less than 10 days to about 80 days. LTX removals of 85 and 95% were achieved by UVC photolysis and UVC/H₂O₂ after 120 min, respectively. Three transformation products, triiodothyronine, diiodothyronine, and diiodotyrosine, were identified during LTX degradation by the UVC-based processes studied. The results herein regarding photo-induced kinetics coupled with environmental fate simulations may help evaluate LTX persistence and also the design of water and wastewater treatment processes.

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