Molecular-level analysis of coagulation mechanism in polyamide and norfloxacin composite system

Microplastics (MPs) and antibiotics will influence each other's removal efficiency during coagulation process. This study investigated the coagulation performance and mechanism of polyamide (PA) (pristine PA (PA-O) and aged PA (PA-A)) and norfloxacin (NOR) in single and composite system. The optimal removal efficiency was achieved 73–90 % for PA-O, 80–92 % for PA-A, 30–41 % (PA-O@NOR) and 23–41 % (PA-A@NOR) for NOR in composite system. Mechanism study found the reaction between oxygen-containing groups and aluminum species was crucial during coagulation process. PA-O could be complexed with Al 3+ through amide group, while PA-A could react with the active groups (η-H 2 O and η-OH) on aluminum hydrolysate species through -OH. As more oxygen-containing groups participated in the reaction, PA-A exhibited advantage of coagulation compared to PA-O. And NOR could either be complexed with Al 3+ or react with the active groups by the oxygen of -C=O and -COOH. Thus, there was a competition between NOR and PA in composite system. Moreover, humic acid (HA) competed with PA and NOR for aluminum hydrolysis products, inhibiting the removal of PA and NOR. This inhibition was more pronounced for PA than for NOR, as HA enhanced the phase transfer of NOR. • Floc mechanism of PA and NOR was discussed on molecular-level analysis. • PA and NOR could compete for Al 3+ , η-H 2 O and η-OH on aluminum species. • PA-A had higher removal efficiency than PA-O due to more oxygen-containing groups. • HA could compete with NOR and PA for the reaction with aluminum species. • Simultaneous removal rates of PA-O and NOR via coagulation were 83.08 % and 47.21 %.