Photogeneration and interactive reactions of three reactive species in the Seto Inland Sea, Japan

Abstract

Environmental contextPhotogenerated reactive species play important roles in the degradation of dissolved organic pollutants. Photogeneration and concerted measurements of hydroxyl (·OH), nitric oxide (NO·) and superoxide (O2·−) radicals in samples from the Seto Inland Sea suggest that their interactive reactions could yield peroxynitrite (ONOO−), a secondary reactive species. These results reveal how discrete photochemical reactions synergise to influence the variety and fates of reactive species in a marine environment. AbstractPhotochemically generated reactive species are involved in photodegradation of dissolved organic pollutants in natural waters. However, there is a dearth of empirical evidence, from each batch of water samples collected, to predict the influence of interactive reactions among several photogenerated reactive species on their variety and fates in natural waters. Concerted photogeneration and measurement of hydroxyl (·OH), nitric oxide (NO·) and superoxide (O2·−) radicals were carried out on water samples obtained during two consecutive summers in 2016 and 2017 from the Seto Inland Sea, Japan. Photogeneration rates of ·OH are (6.98–35.27) × 10−12 M s−1, and those of NO· are (1.20–58.25) × 10−12 M s−1. Compared with these generation rates, that for O2·− ((4.54–18.20) × 10−10 M s−1) was the highest, which suggests that O2·− is a very important photochemically generated reactive species in coastal seawater. The average steady-state concentrations of the three reactive species are ·OH, 7.23 × 10−18 M; O2·−, 3.79 × 10−12 M; and NO·, 1.39 × 10−10 M. Estimated mutual consumption or sink percentages via interactive reactions between O2·− and NO· radicals are five to nine orders of magnitude higher than any other radical pair considered in this study. Hence, we predict that the reaction between photochemical O2·− and NO· could dominate to form ONOO−, a powerful oxidant and nitrating agent, in the coastal marine environment.