This study investigated the degradation and removal of hexazinone, a broad-spectrum triazine herbicide, by advanced water treatments including irradiation with/without hydrogen peroxide (H₂O₂) and adsorption on coconut shell-based granular activated carbon (CSGAC). A comparison between various irradiation sources (visible, UVA, UVB, and UVC) revealed the highest degradation rate under UVC. The experiments were carried out at 0.5 µM initial concentration of hexazinone. The individual efficiency of photolysis (17%; 9 J cm^-2 UV dose) and H₂O₂ (10%; 30 min, 0.5 mM H₂O₂) in degrading hexazinone was inadequate. However, almost complete degradation of hexazinone (>98%) occurred under 3 J cm^-2 UVC dose in the presence of 0.5 mM H₂O₂ at pH 7. Further, the degradation rate increased significantly with an increase in the initial dosage of H₂O₂, UV dose, and contact time in UV/H₂O₂ process. The kinetic studies showed that the degradation followed pseudo-first-order reaction kinetics. In addition, the effect of different scavengers including methanol, potassium iodide (KI), Tert-Butyl alcohol (TBA), and potassium dichromate (K₂Cr₂O₇) showed that hydroxyl radicals (^•OH) are the major reactive species involved in hexazinone degradation. The rate of degradation was lower (0.003 s^-1 µM^-1) using secondary effluent due to the dissolved organics in wastewater than that of deionized water (0.089 s^-1 µM^-1). Approximately 90% adsorption of hexazinone was obtained with 10 h contact time with CSGAC. Despite being hydrophilic, hexazinone was effectively removed by CSGAC, which may be due to its interaction with the functional groups, confirmed through FTIR analysis. Surface functional groups may interact with hexazinone's triazine ring through π–π electron donor-acceptor interactions. The adsorption kinetic of hexazinone onto CSGAC fitted the pseudo-second-order model. This is the first comprehensive study with mechanistic insights on the removal of hexazinone at the tertiary stage of wastewater treatment.

Lokesh Padhye

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