Unraveling the dual roles of dissolved organic matter on the photodegradation of aquatic contaminants: Molecular weight- and type-dependent heterogeneities

Abstract

Dissolved organic matter (DOM) in natural waters can regulate the behaviors and fates of aquatic contaminants, while the specific effects on contaminant attenuation are highly dependent on its inherent properties [e.g., molecular weights (MW) and types]. In this study, the algae-derived organic matter (AOM) and humic acid (HA) were selected as the representative autochthonous and allochthonous DOMs, which were further fractionated into low MW (LMW, <1 kDa) and high MW (HMW, <1 kDa~0.45 μm) fractions to evaluate the MW- and type-dependent heterogeneities in the photodegradation of sulfadiazine (SDZ). Results showed that presence of bulk AOM promoted SDZ photodegradation by 2.45 folds while those of the bulk HA inhibited SDZ photodegradation by 1.70 folds due to the higher light screening effects and phenolic antioxidant concentrations. Further analysis revealed obvious MW-dependent heterogeneities that, regardless of DOM types, the HMW-fraction always inhibited SDZ photodegradation while the LMW-fraction promoted photodegradation efficiencies owing to higher carbonyl contents and electron transfer capabilities. In addition, the MW-dependent heterogeneities within DOM samples resulted in different photodegradation pathways and Ecological Structure-Activity Relationship (ECOSAR) calculation showed that most of photodegradation products in the LMW-fraction were more ecotoxicity than the parent SDZ while those in the HMW-fraction exhibited alleviated ecotoxicity. This study indicated that the dual roles of aquatic DOMs on contaminant photodegradation were MW- and type-dependent, and detailed structural composition analysis on DOM matrix was needed for a better assessment of the behaviors and fates of contaminants in aquatic ecosystems.