Carboxylated graphene quantum dots as emerging precursors of disinfection byproducts: Mechanistic insights into chlorine-driven transformation and environmental risk amplification

Carboxylated graphene quantum dots (cGQDs), emerging from diverse industrial applications, pose significant environmental risks as precursors of disinfection byproducts (DBPs) in water treatment. This study systematically investigates the transformation mechanisms and DBPs formation potential of cGQDs during chlorination and chloramination. cGQDs undergo disruption of π-conjugated structures and covalent halogenation through direct chlorination and indirect radical-mediated (•OH, Cl•, ClO•), resulting in substantial DBPs generation. Notably, trichloromethane (TCM) concentrations reached 146.47 µg/L (low chlorine) and 697.44 µg/L (high chlorine). Particularly under low chlorine conditions, which represent concentrations typical of municipal wastewater disinfection, cGQDs produced significantly higher TCM than conventional carbon materials at equivalent concentrations, exceeding those of graphene oxide and graphene by 3-fold and 29-fold, respectively. This enhanced reactivity is attributed to nanoscale dimensions and carboxyl-rich surfaces of cGQDs. In contrast, although chloramination can reduce the generation of DBPs, it may lead to more severe environmental impacts, such as the formation of nitrogen-doped GQDs (N-GQDs) with a narrower bandgap, which can complex with metal ions like iron(III) and subsequently affect water quality. Real-water experiments demonstrated that while natural organic matter partially suppresses DBPs formation from cGQDs via radical scavenging, cGQDs still increased TCM formation by 20.8 % in surface water and 21.3 % in wastewater. These findings highlight the unique reactivity of cGQDs as DBPs precursors, thereby providing critical insights for refining disinfection strategies and managing nanomaterial-related hazards in water treatment.