Characterization of dissolved black carbon and its binding behaviors to ceftazidime and diclofenac pharmaceuticals: Employing the molecular weight fractionation

As the ubiquitous component of the aquatic environment, dissolved organic matter (DOM) readily bind with residual pharmaceutical contaminants (PCs) and influence their environmental behaviors. However, the binding mechanisms between dissolved black carbon (DBC), a vital part of the natural DOM pool, and PCs were poorly researched. In this study, the bulk DBC was divided into four fractions in molecular weight (MW) via an ultrafiltration system, and the properties of DBC and their binding interaction with two kinds of typical PCs (ceftazidime (CAZ) and diclofenac (DCF)) were explored concretely. The results showed that low MW component was the main contributor to bulk DBC, and the aromaticity increased with the increase of MW. The categories of chemical structures and fluorescent substances in different MW DBC were similar. Multispectral techniques showed that the oxygen-enriched compounds in DBC had the higher affinity to CAZ/DCF. The -NH-, -COOH, -NH2 groups in CAZ molecules appeared to form the hydrogen bond with DBC. Fluorescence quenching experiments were analyzed, and the binding mechanisms were specifically expounded from the thermodynamic perspective. The fluorophore of fulvic acid-like compounds (FA) were quenched by both static and dynamic quenching mechanisms, while only static quenching occurred for humic acid-like compounds (HA). For bulk DBC, the hydrogen bond and van der Waals force were the major forces in the HA-CAZ system, while the hydrophobic force made the primary contribution to the HA-DCF system, which might be ascribed to the higher hydrophobic nature of DCF. Notably, with the increase of HA MW, the main binding mode of HA-CAZ/DCF changed from hydrophobic force to hydrogen bond and van der Waals force gradually, which also directly proved that various noncovalent interactions co-driven the binding processes. Our findings are beneficial to better assess the fate of DBC and PCs and the corresponding complexes in the aquatic environment.