Hydroxyl radical-mediated degradation of diclofenac revisited: a computational approach to assessment of reaction mechanisms and by-products
Advanced oxidation processes (AOPs) are based on the in situ production of hydroxyl radicals (•OH) and reactive oxygen species (ROS) in water upon irradiation of the sample by UV light, ultrasound, electromagnetic radiation, and/or the addition of ozone or a semiconductor. Diclofenac (DCF), one of the emerging organic contaminants (EOC), is of environmental concern due to its abundancy in water and is known to be subjected to AOPs. The current study uses density functional theory (DFT) to elucidate the mechanisms of the reactions between •OH and DCF leading to degradation by-products, P1-P9. The initial encounter of DCF with •OH is proposed to lead to either the abstraction of a hydrogen or the addition of the hydroxyl radical to the molecule. The results showed that OH addition radicals (Radd) are both kinetically and thermodynamically favored over H abstraction radicals (Rabs). The intermediate radicals give degradation by-products by subsequent reactions. The by-products P7 and P8 are easily formed in agreement with experimental findings. Finally, acute toxicities at three trophic levels are estimated with the Ecological Structure Activity Relationships program. DCF and most of the by-products were found to be harmful to aquatic organisms, P9 being the only by-product that is not harmful at all three trophic levels.
KeywordsDiclofenac Degradation Toxicity Mechanism DFT Hydroxyl radical Sonolysis
- ECOSAR, (2014), http://www.epa.gov/oppt/newchems/tools/21ecosar.htm,
- Frisch MJ et al (2009) Gaussian 09. Gaussian Inc., WallingfordGoogle Scholar
- Li GY, Nie X, Gao YP, An TC (2016) Can environmental pharmaceuticals be photocatalytically degraded and completely mineralized in water using g-C3N4/TiO2 under visible light irradiation?-implications of persistent toxic intermediates. Appl Catal B Environ 180:726–732. doi:10.1016/j.apcatb.2015.07.014 CrossRefGoogle Scholar
- Mennucci B, Cances E, Tomasi J (1997) Evaluation of solvent effects in isotropic and anisotropic dielectrics and in ionic solutions with a unified integral equation method: theoretical bases, computational implementation, and numerical applications. J Phys Chem B 101:10506–10517. doi:10.1021/jp971959k CrossRefGoogle Scholar
- Zhao Y, Truhlar DG (2004) Hybrid meta density functional theory methods for thermochemistry, thermochemical kinetics, and noncovalent interactions: the MPW1B95 and MPWB1K models and comparative assessments for hydrogen bonding and van der Waals interactions. J Phys Chem A 108:6908–6918. doi:10.1021/jp048147q CrossRefGoogle Scholar