Solar photolytic ozonation for the removal of recalcitrant herbicides in river water

Abstract

Photolytic ozonation of a river water has been performed by means of simulated solar radiation. The application of solar radiation, limiting the complete radiation spectrum (300–800 nm) to 320–800 nm and 390–800 nm, during the aqueous ozone decomposition has been assessed. A kinetic mechanism, including the influence of initiation, promotion and scavenging substances has been proposed, successfully modeling the experimental data. Radiation improves O3 decomposition rate, as a promoter, being higher if the complete UV–visible spectrum is applied. Also, pH positively influences O3 decomposition rate from pH = 4 to 8.

Photolytic ozonation has been also proved to be effective in the removal of a mixture of three pyridine herbicides, dissolved in the river matrix. Radiation filters (320 nm and 390 nm cut-off) and pH have been selected as the main variables of the study. The enhanced oxidation rate registered when applying solar radiation and O3 relays on the higher formation of hydroxyl radicals, responsible for the oxidation of these recalcitrant-to-ozone herbicides. Moreover, the estimated RCT ratios confirmed the minimal differences of applying radiation or not at increasing pH, what is due to the ability of hydroxide anion to catalyze the decomposition of O3 into HOradical dot. The mineralization of the photolytic ozonation process (300–800 nm) reached 60%, whatever the pH considered. The increase of pH minimizes the differences in mineralization between the two technologies, the single ozonation achieving 50% at alkaline pH.