Interactions between dissolved organic matter of different molecular weights and hexabromocyclododecanes

Although dissolved organic matter (DOM) has been reported to interact with various persistent organic pollutants (POPs), the molecular-weight-dependent interactions between DOM and hexabromocyclododecanes (HBCDs) remain poorly characterized. In particular, limited information is available on how DOM molecular weight regulates the binding affinity and fluorescence response of individual HBCD isomers, including α-, β-, and γ-HBCD. To elucidate the interactions between DOM of varying molecular weights and HBCDs, this study employed Fulvic Acid (FA), Humic Acid (HA), and Suwannee River Natural Organic Matter (SRNOM) as representative dissolved organic matter. Using ultrafiltration, ultraviolet-visible (UV-Vis) spectroscopy, three-dimensional excitation-emission matrix combined with parallel factor analysis (EEM-PARAFAC), and Ultra Performance Liquid Chromatography-Tandem Mass Spectrometry (UPLC-MS/MS) to characterize the relationships between HBCDs and DOM of different types and molecular weights. The results indicate that HBCDs addition affects DOM across all types and molecular weight ranges (30 KDa ∼0.45 μm, 10 ∼ 30 KDa, 1 ∼ 10 KDa, and <1 KDa). However, the interaction mechanism is complex and lacks uniform patterns, manifesting as fluorescence response (quenching or enhancement) varies with DOM type, molecular weight, and HBCDs concentration, exhibiting non-linear relationships. The apparent binding constants between the four molecular-weight fractions and HBCDs, as well as their individual isomers, decreased with increasing DOM concentration, with the highest binding constants observed at low DOM concentration. High-molecular-weight FA and HA exhibited stronger binding capacities, whereas SRNOM showed the highest binding constants at low concentrations for low-molecular-weight components. The binding constants of γ-HBCD with FA, HA, and SRNOM were significantly higher than those with other isomers, suggesting γ-HBCD have higher hydrophobicity and a stronger binding capacity with organic matter. This binding state may also directly influence its accumulation within organisms and potential bioavailability.