Photodegradation and adsorption of hexazinone in aqueous solutions: removal efficiencies, kinetics, and mechanisms

Hexazinone, a globally applied broad-spectrum triazine herbicide, has not been mechanistically investigated previously under advanced oxidation processes (AOPs) and adsorption on activated carbon. In this study, its fate during UV-based oxidation with/without hydrogen peroxide (H₂O₂) and adsorption on coconut shell-based granular activated carbon (CSGAC) in water matrices was investigated. A comparison between various irradiation sources (visible, UVA, UVB, and UVC) revealed the highest degradation rate under UVC. More than 98% degradation of hexazinone was observed under 3 J cm^-2 UVC fluence in the presence of 0.5 mM H₂O₂ at pH 7. Moreover, the degradation rate enhanced significantly with an increase in the initial dosage of H₂O₂, UV fluence, and contact time in the UV/H₂O₂ process. The rate of degradation was lower using secondary effluent than that of Milli-Q water due to the presence of dissolved organics in wastewater. However, the reactions in both matrices obeyed pseudo-first-order kinetics. The effect of different scavengers, including methanol, potassium iodide (KI), and tert-butyl alcohol (TBA), showed that hydroxyl radicals (^•OH) played a dominant role in hexazinone degradation in the UV/H₂O₂ process. Hexazinone was effectively adsorbed by CSGAC through π-π electron donor-acceptor interactions between hexazinone's triazine ring and CSGAC's surface functional groups. The isotherm and kinetic studies showed that the adsorption followed the Freundlich model and pseudo-second-order reaction, respectively, suggesting chemisorption. This study provided mechanistic insights on the removal of hexazinone at the tertiary stage of wastewater treatment or the advanced treatment of wastewater reuse.