Hydroxyl radical-mediated degradation of diclofenac revisited: a computational approach to assessment of reaction mechanisms and by-products

Abstract

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 (R ) are both kinetically and thermodynamically favored over H abstraction radicals (R). 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.

Cephalexin Degradation Initiated by OH Radicals: Theoretical Prediction of the Mechanisms and the Toxicity of Byproducts

“…for CLX, and this value is in good correlation with that reported in the literature obtained experimentally (0.65) [28,31].…”

“…The reactivity of cephalexin toward •OH is supposed to be either by hydrogen abstraction or hydroxyl addition. The concerned sites on cephalexin molecules are either hydrogen atoms, in which the outcome of their interaction with hydroxyl radicals is an abstraction reaction, or unsaturated carbon atoms, in which the result of their interaction with hydroxyl radicals is an addition reaction [28]. The reactivity of the candidate sites is studied through atom condensed Fukui index analysis.…”

“…The toxicities of degradation byproducts were classified according to the Globally Harmonized System of Classification and Labeling of Chemicals (GHS) [28], as shown in Table 9. The toxicity was measured for the degradation byproducts (P1-P7) as well as the starting molecule (cephalexin); the results are presented in Table 10.…”

Effects of HCO3– on Degradation of Toxic Contaminants of Emerging Concern by UV/NO3–

“…Cyclization product D 260 was detected and identified during DCF decomposition in UV/NO 3 – /HCO 3 – . This route was initiated by • OH-induced H-abstraction at C 8 as illustrated in SI Figure S17c. , The formed radical anion subsequently removed a chloride anion at C 6 via the dechlorination route to form a biradical under UV irradiation, which leads to a quick recombination with the generation of a five-membered cyclic product. Decarboxylation that led to the generation of D 216 , could be initiated with • OH-involved electron transfer on the carboxylic acid of the D 260 , which first formed a carboxylic radical, followed by removal of CO 2 and electron .…”

Protective Effect of Selected Antioxidants on Naproxen Photodegradation in Aqueous Media

“…It is reported that the biological effects of some pharmaceuticals are changed by UV irradiation [6,7,8] on account of their photoconversion to photoproducts. Various photochemical reactions may be induced by UV irradiation [9,10,11]. These reports indicate that the generation of various photoproducts is dependent on these chemical reactions.…”