A new molecular weight (MW) descriptor of dissolved organic matter to represent the MW-dependent distribution of aromatic condensation: Insights from biodegradation and pyrene binding experiments

In this study, we utilized a size exclusion chromatography (SEC) system that was equipped with a fluorescence emission scanning mode to explore the heterogeneous distribution of the humification index (HIX) values within bulk dissolved organic matter (DOM). The HIX-based SEC chromatograms showed that the molecular weight (MW)-dependent distribution of aromatic condensation was heavily affected by the DOM sources and the progress of biodegradation. The HIX heterogeneity across different MW was more pronounced for terrestrial versus aquatic DOM sources. Microbial incubation of leaf litter DOM led to the initial enhancement of the HIX at a relatively low MW, followed by a gradual increase at larger MW values. The dynamic changes of the HIX can be attributed to (1) the preferential removal of non-aromatic or less-aromatic molecules by microorganisms, (2) the production of microbial metabolites, (3) microbial humification, and (4) self-assemblage of humic-like molecules. From the SEC chromatograms, the HIX-based average molecular weight (or MWHIX) was proposed as a unifying surrogate to represent an MW that was highly associated with aromatic condensation. The MWHIX discriminated four different DOM sources and described well the biodegradation-induced changes. The MWHIX also presented a good positive correlation with pyrene organic carbon-normalized binding coefficients (Koc). The prediction capability of the MWHIX for pyrene Koc was higher than those based on the single descriptors of bulk DOM, such as HIX and MW, which revealed its superior linkage with the DOM reactivity related to both MW and HIX.