Petrogenic polycyclic aromatic sulfur heterocycles (PASHs) are introduced in natural waters through crude oil spills, run off from petroleum treated areas, emissions from diesel fuels or coal power plants followed by subsequent wet or dry deposition. Degradation mediated by sunlight has been suggested as a mechanism removing major components of spilled oil, and may be important for PASHs in shallow aquatic ecosystems. Alkylated derivatives of PASHs have been shown to have faster direct photodegradation rates. However, limited information is available on aquatic indirect photodegradation pathways. Dibenzothiophene (DBT) and its homologous, C1-DBT, C2-DBT and C4-DBT, were chosen as model sulfur heterocycles to investigate the impact of molecular structure on rate of reaction with the photochemically produced oxidant, hydroxyl radical. The hydroxyl radical rate constant (K.OH) of PASHs was measured by a competition kinetic method. The analogue C1-DBT reacted fastest relative to other PASHs with respective mean measured rate constants (× 10-10) ± the standard deviation (× 10-10) of 1.2 ± 0.2, 2.1 ± 0.5, 0.5 ± 0.1, and 0.9 ± 0.1 M-1 S-1 for DBT, C1-DBT, C2-DBT and C4-DBT, respectively. Steric hindrance by alkyl substituents on the sulfur moiety most strongly impacted measured rate constants between DBT and individual alkyl derivatives. The EPA Exposure Analysis Modeling System (EXAMS) was used to estimate the impact of this transformation process on overall chemical half-life as a function of depth (0.1, 1 and 2 m). Indirect photodegradation rates for the less polar C2-DBT and C4-DBT were slower than DBT and C1-DBT at all depths. Overall, the PASH estimated half-lives were longer at increased depths due to lower steady-state concentration of photochemically produced hydroxyl radical as a result of light attenuation.

Steric hindrance reduces aquatic photochemical transformation rates of alkylated sulfur heterocycles

253rd National Meeting (2017)

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