Oxidative Degradation of Betrixaban and Hexazinone: Kinetics, Reaction Mechanisms, and Nitrogenous Disinfection By-Products Formation Potential

Abstract

The objective of this study was to investigate the removal mechanism of betrixaban, an oral anticoagulant drug approved by the United States Food and Drug Administration (U.S. FDA), and hexazinone, a broad-spectrum triazine herbicide, at oxidation and adsorption stages of water/wastewater treatment. Their N-nitrosodimethylamine (NDMA) formation potential in the presence of monochloramine (NH₂Cl), free chlorine, ozone (O₃) and ultraviolet (UV)-based oxidation with/without hydrogen peroxide (H₂O₂) was also investigated. The novel analytical method for simultaneous detection and identification of betrixaban and hexazinone by liquid chromatography/tandem mass spectrometry (LC-MS/MS) using multiple reaction monitoring (MRM) in positive electrospray ionization (ESI) mode was also developed, optimised, and validated.The oxidative degradation results revealed that an increase in NH₂Cl dosage, contact time, and pH significantly increased betrixaban degradation and NDMA formation, which exceeded 1% at basic pH upon chloramination. Moreover, betrixaban showed a complete degradation with 5 mM free chlorine over seven days with 0.01% NDMA yield. The degradation of betrixaban, in the presence of 0.3 mM O₃, increased from 40% after 1 minute to almost complete degradation after 6 minutes of contact time, with no observed NDMA formation. The kinetic studies revealed that the reaction of betrixaban with NH₂Cl and O₃ followed pseudo-first-order reaction kinetics, while no reaction kinetics was obtained for chlorination of betrixaban due to its complete removal within the initial two minutes of the reaction. Monochloramination of betrixaban also led to the formation of other disinfection by-products (DBPs) such as dichloroacetonitrile (DCAN) and dimethylformamide (DMF). DMF was also detected as a prominent DBP of chlorination and ozonation of betrixaban. As the use of betrixaban will widen, leading to its increased presence in the environment, wastewater and surface waters, its DBPs’ formation risks in the environment will also increase. None of the above-mentioned oxidants could form NDMA from the oxidation of hexazinone. UVC irradiation (3 J cm⁻²), in combination with H₂O₂ (0.5 mM), led to >90% degradation of betrixaban and hexazinone at pH 7. A comparison between various irradiation sources, visible, UVA, UVB, and UVC, revealed the highest degradation rate of betrixaban and hexazinone under UVC in the presence of H₂O₂. An increase in the initial H₂O₂ dosage, light intensity, and contact time enhanced the degradation rates of betrixaban and hexazinone. The kinetic studies showed that the degradation of these compounds followed pseudo-first-order reaction kinetics. Experiments using different scavengers demonstrated hydroxyl radicals (•OH) as the major reactive species involved in degradation during the UV/H₂O₂ process. In the last phase of the study, photocatalytic degradation of hexazinone, resistant to photolysis, was investigated using ferric chloride (FeCl₃) and a conducting polymer, poly(3, 4-ethylenedioxythiophene) (PEDOT). Enhanced degradation of hexazinone was observed under irradiation by increasing the concentrations of FeCl₃. Moreover, PEDOT was successfully immobilised on the surface of carbon fibre cloth through electrochemical polymerisation. The photocatalyst demonstrated approximately 80% removal of hexazinone through a combination of adsorption and photocatalysis. The reaction mechanism was elucidated using methanol, showing the important role of •OH in photodegrading hexazinone in the presence of FeCl₃ and immobilised PEDOT individually. Finally, it was revealed that the addition of FeCl₃ could boost up the performance of PEDOT on the photodegradation of the contaminant due to its additive effect.