Theoretical insights into the transformation mechanism and eco-toxicity effects of 5-Fluorouracil by O3 and ·OH in waters

The existences of non-biodegradable 5-Fluorouracil (5-FU) in aquatic systems and its potential toxic effects on aquatic organisms have caused widespread concern. In this work, two typical oxidants (O3 and ·OH) were selected to investigate the mechanism, kinetics, and the potential eco-toxicology assessment of 5-FU using computational chemistry methods. Results show that 5-FU can be degraded rapidly by O3 and ·OH, which subsequently undergoes ring-opening, decomposition, defluorination, and hydroxylation steps. The rate constants of initiation reactions are suppressed as the temperature increases. The half-lives of 5-FU determined by O3 and ·OH are on the order of seconds in the advanced oxidation processes (AOPs) system. Fifteen structures of transformation products were identified by theoretical calculation, including five experimental products. The toxicity assessment results show that the acute and chronic toxicities of the degradation process to aquatic organisms gradually decreased, but the developmental toxicity and mutagenicity of several products on human still exist. In addition, the main products have been found to decompose into some small molecules (NO, Formic acid, Acetic acid, etc.). This result could help to reveal the transformation behaviors and risk assessment of 5-FU in aquatic environments, and further design the experimental and industrial infrastructure.