Using compound-specific isotope analysis to identify the mechanism of acetochlor degradation during oxygenation of hyporheic zone sediment

Biodegradation is recognized as the main pathway for acetochlor attenuation in aquatic environments. However, the potential abiotic degradation of acetochlor by hydroxyl radicals (center dot OH) generated during oxygenation of hyporheic zone sediments has not been investigated. This study aims to examine the production of center dot OH during oxygenation of hyporheic zone sediments and its effects on acetochlor attenuation. A significant decrease of acetochlor, ranging from 77.9% to 100%, was observed in the water-sediment systems with extensive center dot OH production. The primary sources of center dot OH production were found to be the oxidation of Fe(II) and reduced humic acids. Furthermore, a center dot OH quenching experiment suggests that center dot OH driven oxidation is the dominant pathway for acetochlor attenuation. Carbon isotope fractionation of acetochlor degradation during oxygenation of sediments (epsilon bulk,C ranged from - 1.5%0 to -0.5 +/- 0.3%0) was close to that during acetochlor degradation by center dot OH in a H2O2-Fe3O4 Fenton system (epsilon bulk,C = -0.5 +/- 0.1%0), but significantly smaller than that during acetochlor biodegradation (epsilon bulk,C = -5.8 +/- 0.9%0). Compound-specific isotope analysis (CSIA) further suggests that center dot OH produced by sediment oxygenation plays a critical role in acetochlor attenuation in aquatic environments. Results of calculated apparent kinetic isotope effect of carbon (AKIEC) and transformation products indicate that SN1 and SN2-type nucleophilic substitution are the first steps in acetochlor attenuation through center dot OH driven oxidation (AKIEC = 1.007 +/- 0.001) and aerobic biodegradation (AKIEC = 1.088 +/- 0.013), respectively. Our findings highlight the potential of CSIA to assess the acetochlor degradation in water-sediment system, which can help to elucidate the fate of herbicide in aquatic environments.