Environmental fate of phenobarbital in water: Homogeneous and non-homogeneous environment

Abstract

In aqueous environments, phenobarbital (PB) undergoes degradation mediated by hydroxyl radicals (HO·) and sulfate radicals (SO4·−). In this study, a theoretical approach was used to investigate the oxidation of phenobarbital triggered by HO·/ SO4·− in different dissociated forms of PB at different pH values, as well as the adsorption mechanism of phenobarbital on the silica surface and the non-homogeneous reaction with HO·. Our study confirms three primary reaction models for PB degradation: radical addition, hydrogen abstraction, and single electron transfer reactions. Thermodynamic and kinetic calculations show that the reaction of PB with HO· is the main degradation pathway compared to the reaction of SO4−. At the C5 and C6 positions, HO·-addition and H-abstraction from H6 and H7 are competitive mechanisms. However, in non-homogeneous environments, HO·-addition at C5 becomes the dominant pathway. Our work identifies the primary products of these degradation reactions. At a temperature of 298 K, the total rate constants for the degradation reactions of HO· with PB neutral molecules and anions were determined to be 2.06 M−1 s−1 and 4.11 × 107 M−1 s−1, respectively. In a non-homogeneous reaction with HO·, the total rate constant is 4.47 × 1010 M−1 s−1. Despite degradation, the majority of these products retain their potential to impact aquatic organisms. In particular, compounds such as 5-ethyl-5-hydroxypyrimidine-2,4,6(1H,3H,5H)-trione, 5-(1-hydroxyethyl)-5-phenylpyrimidine-2,4,6(1H,3H,5H)-trione, and 5-acetyl-5-phenylpyrimidine-2,4,6(1H,3H,5H)-trione have a greater toxic effect on green algae. The findings underscore the need for future research to consider the environmental implications and ecotoxicity of PB. Such consideration is crucial for a comprehensive understanding of the long-term environmental impact of pharmaceutical residues in aquatic ecosystems.