An ultrasound-assisted photocatalytic treatment to remove an herbicidal pollutant from wastewaters

The EU Water Framework Directive on Environmental Quality Standards 2008/105/EC announced in Annex X a list of 33 priority substances which includes metals, pesticides, phthalates, polycyclic aromatic hydrocarbons and endocrine disruptors [1]. While the occurrence and effects of metals, bacteria, hydrocarbons and other ions like nitrates and ammonia in water are extensively described, data on pesticides, pharmaceuticals and phthalates are seldom available. Specifically, herbicides are more water soluble, polar and thermally stable than other contaminants. They have harmful effects on soil, flora and fauna, surface and ground water, which will eventually affect the human being and livestock [2]. The problem of emerging pollutants is the lack of knowledge of their impact on both human health and environment upon long and short-term exposure [3]. Isoproturon is a phenylurea herbicide with proven subacute toxicity on rats [4] as well as endocrine disruptor capability on humans [5]. It promotes tumor growth and accounts for other ailments related to the reproductive system [6]. Notwithstanding TiO2 based catalysts are efficient in terms of wastewater organic pollutants removal, the formation of toxic by-products is to be taken into account [7]. Ultrasound may intensify the photocatalytic wastewater treatment. When applied to a liquid, ultrasonic waves generate vapour-filled voids via a cyclic succession of expansion and compression phases. Upon collapsing, each cavitation bubble acts as hotspot by generating micro-jets whose energy increases the temperature and pressure up to 5000 K and 500 atm, respectively [8]. These conditions cause water to split and hydroxyl radicals formation, which further assist TiO2 photocatalysts in a coupled Advanced Oxidation Process. In the present work, we investigate the ultrasound assisted photocatalytic degradation pathway of the herbicide Isoproturon (IPU). We worked in batch mode in a thermostatic glass reactor. We compared the activity of P25, a nanometric 80 % anatase and 20 % rutile highly active standard [9], with that of Kronos 1077, a micrometric 100 % anatase TiO2 catalyst [10] whose particle size makes its filtration easier. They were dispersed in a synthetic solution of IPU (20 ppm). An external UV A lamp (365 nm) irradiated the reactor sideways with 15 mW cm-2. A 20 kHz ultrasonic horn delivered 15 Watt to the solution. With a concentration of 0.1 g L-1, in 6h Kronos degrades the 93 % of Isoproturon whereas P25 converted 99 % of the pesticide. Ultrasound alone converts the 45 % of IPU in 6h. We monitored the reaction via HPLC-UV at 240 nm and 0.5 mL/min using acetonitrile and water (1:1) as mobile phase. In the near future, we will investigate the effect of UV light when coupled with ultrasound on the degradation rate of IPU, as well as the toxicity of the wastewater after treatment. We will establish a degradation pathway after identifying the degradation by-products via HPLC-MS.