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

Lokesh P. Padhye,Tahereh Jasemizad

doi:10.1007/s11356-022-19205-y

Lokesh P. Padhye, Tahereh Jasemizad

Open Access

https://doi.org/10.1007/s11356-022-19205-y

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Abstract

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.

Highlights

Over the past few decades, pesticides have been extensively applied in intensive agriculture practices, which has substantially contributed to an increase in the pollution of surface water and groundwater (Adak et al 2019; Padhye et al 2013)
Since the highest hexazinone degradation was observed under UVC irradiation, with and without H2O2, the rest of the experiments were performed under UVC (254 nm)
The results indicated that the photodegradation rate of hexazinone could be enhanced by the addition of H 2O2 in the presence of UV, under UVC irradiation, due to the formation of more OH from H2O2

Summary

Introduction

Over the past few decades, pesticides have been extensively applied in intensive agriculture practices, which has substantially contributed to an increase in the pollution of surface water and groundwater (Adak et al 2019; Padhye et al 2013). Hexazinone, a globally used broad-spectrum triazine (Mei et al 2012; Yao et al 2019), is considered the most water-soluble triazine herbicide (33 g/L) and is transported to groundwater and surface water near agricultural areas, where it is widely applied (Ganapathy 1996; Martins et al 2015). It was first registered by the United.

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