Carbon Isotope Fractionation of Dissolved Organic Matter Due to •OH-Based Oxidation.

•OH-based oxidation plays a crucial role in dissolved organic matter (DOM) transformation and carbon flux, whereas quantifying the contribution of this pathway remains challenging. Here we combined the concentration with the carbon isotope analysis of DOM and its generated CO2 to quantify the contribution of •OH-based oxidation. Results showed that the 13C enrichment factors (ε values) were -8.1‰ to -8.9‰ for benzene ring oxidation in aromatic compounds, -4.2‰ to -28.9‰ for lower-molecular-weight organic acids, and -13.0‰ for DOM from sediment. The fractionation of sediment DOM reflects the average ε value of humic substances and organic acids. These ε values were more negative than those of the photochemical and microbial processes, enabling the identification of DOM transformation mechanisms. Using an end-member mix model, we found that the proportion of •OH-based mineralization in total CO2 emission ranged from 20.9% to 39.8% for 100 g/L sediment oxidation by 5-20 mM H2O2 under pH-neutral condition within 2 h and was only 2% for oxidation by air under the same conditions. We also found that inorganic carbon degassing contributed greatly to CO2 emission during sediment oxidation. This study presents a new isotope-based tool to quantitatively assess the contribution of •OH-based oxidation to the emission of CO2 from DOM.