Photogeneration and steady-state concentration of hydroxyl radical in river and lake waters along middle-lower Yangtze region, China

Hydroxyl radical (HO∙) in natural waters plays a critical role in contaminant transformation and ecosystem health. In this study, the photogeneration and steady-state concentration of HO∙ in different aquatic environments (e.g., river and lake) along the middle-lower Yangtze region, China, were evaluated. The results showed that, compared to lake samples, the river waters were characterized by lower HO∙ photoformation rate (RHO∙) (5.10–11.69 × 10−11 vs. 1.10–1.82 × 10−10 M s−1) and steady-state HO∙ concentration ([HO∙]) (1.76–3.11 × 10−17 vs. 2.50–10.33 × 10−17 M). The contribution of nitrate and nitrite to the total RHO∙ in river waters was generally higher than that in lake waters, and photolysis of nitrite exhibited contributions 1–2 times higher than those of nitrate (0–25% vs. 0–9%) irrespective of sample types. As a result, the photosensitization by chromophoric dissolved organic matter (CDOM) contributed more than 70% of the total RHO∙ for all samples except for River Ganjiang. [HO∙] among all samples was positively correlated with dissolved organic carbon (DOC) concentration, and the DOC-normalized [HO∙] was further related to the physicochemical properties of CDOM samples (e.g., aromaticity, humification, and molecular weight). The humic-like aromatic substances with low molecular weight were the controlling factors influencing [HO∙] in the studied surface waters. The results contributed to a deeper understanding of behaviors and fate of aquatic DOMs in terms of HO∙ formation and contaminant attenuation.