Singlet oxygen and hydroxyl radical in the transformation of acetaminophen in surface water: The mechanism on the degradation pathways based on DFT calculations

Abstract

Pharmaceuticals compounds are continuously released into the aquatic environment due to the widely used therapeutically in livestock and humans. These pharmaceuticals and their degradation products accumulate in fish and have potential impacts on human and ecosystems. In this study, acetaminophen (ACE), a pharmaceutical widely used in COVID-19 epidemic, was selected as the research object to study its microscopic transformation mechanism and ecotoxicity using density functional theory and computational toxicology. The research demonstrated that ACE could be degraded by 1O2 in three pathways (2,2-cycloaddition, Ene reaction and 2,4-cycloaddition). The 2,4-cycloaddition mechanism at para-site plays a dominant role in the 1O2 oxidation of ACE. The total rate constant of ACE for •OH-initiated reaction was 1.78×109 M−1s−1 at 298 K. Furthermore, the half-life of ACE was determined as 143 and 3.5 days at ambient temperatures, given the concentration of •OH and 1O2 as 10−16.5 and 10−13 M, respectively. Although the aquatic toxicity of most degradation by-products appears to be not harmful level, some products were identified as very toxic and have developmental toxicity, especially hydroquinone and 1,4-benzoquinone. The systematic mechanism study provides a pathway to understand the transformation and environmental impact of pharmaceutical pollutants containing paracetamol in natural water environment at the molecular level.