Nitrate photosensitized degradation of atrazine during UV water treatment

Abstract

Increasingly, ultraviolet radiation (UV) is being used to treat chemical contaminants in waste and drinking water. To understand the significance of indirect photolysis during such processes, the effect of nitrate on UV treatment of atrazine (ATZ) was investigated at pH 7 in phosphate buffer and solutions containing Suwannee River natural organic matter (NOM). With a medium-pressure mercury lamp, environmentally relevant levels of nitrate (0.7–7 mg-N L–1) lead to initial rates of ATZ removal that are enhanced compared to those without nitrate, and comparison of the product distributions obtained with nitrate and with hydrogen peroxide indicates that this can be attributed to hydroxyl radical (•OH) production from nitrate absorption below 250 nm. Consistent with this, the observed rates decrease as the reaction progresses, presumably due to •OH scavenging by photochemically generated nitrite. In solutions containing NOM, this effect is not observed, and the addition of nitrate leads to lower rates of ATZ removal, albeit higher than predicted on the basis of light-screening by nitrate alone. In order to model this effect, the •OH quantum yield from nitrate photolysis below 250 nm was determined by two methods, competition kinetics between ATZ and 1-octanol and formaldehyde production by reaction of •OH with methanol, and it was found to lie in the range 0.09 to 0.14. This is the first report of the quantum yield at these wavelengths, and this information allows the photolysis rates to be modeled as a function of nitrate concentration and water quality.