Photo-production of excited triplet-state of dissolved organic matters in inland freshwater and coastal seawater

Abstract

The excited triplet-state of dissolved organic matter (³DOM*) is a major reactive intermediate in sunlit waters. Its quantum yield is important in understanding the fate of organic micropollutants. The degradation efficiency of its chemical probe, 2,4,6-trimeythlphenol (f_TMP), is generally used as a proxy of the quantum yield. However, f_TMP has been described and modelled only for freshwater systems. Therefore, this study quantified f_TMP in inland freshwater and coastal seawater sampled in Japan by conducting steady-state photochemical experiments. Optical properties of water were then used to model f_TMP. Results indicated that the inland freshwater DOM originated mainly from terrestrial sources, while the coastal seawater DOM were microbial-dominated. On average, inland freshwater exhibited lower f_TMP (61.2 M^-1) than coastal seawater (79.7 M^-1) and the coastal seawater exhibited significant variations in the proportion of high-energy ³DOM* (> 250 kJ/mol). In addition, E₂:E₃ (ratio of absorbance at 254 to 365 nm) was positively correlated with f_TMP of inland freshwater, coastal seawater, and the overall dataset. Catchment conditions such as forest coverage also influenced the production of ³DOM* and high-energy ³DOM* in inland freshwater. Furthermore, the developed models estimated f_TMP based on the optical properties of both freshwater and seawater, providing valuable insights about ³DOM* photochemistry in the aquatic environment.

Keywords: High-energy triplets; Modeling; Optical property; Photochemistry; Quantum yield coefficient.