Synergy of electrostatic interaction and bridge effect for the selective co-removal of Cr(VI) with high ionization degree of antibiotics by Fe3C-carbon nanohybrids.

Co-removal of Cr(VI) and antibiotics is challenging and the microscopic mechanisms about their interplay at heterogeneous interfaces remain unexplored. Here, we report a Fe3C-carbon nanohybrids of biochar-carbon nanotube composite (MBCFe) that enables Cr(VI)-triggered cationic antibiotic removal without external energy input. Electrostatic attraction mediated the increased removal of positive charged enrofloxacin (ENR) on negative charged MBCFe3 when Cr(VI) existed, but that of amphiphilic amoxicillin reduced. X-ray absorption near edge structure spectra revealed Cr(VI) adsorption induced electron transfer to produce Cr(OH)2+, Cr3+, FeCr2O4 and (CrxFe1-x)(OH)3. Density functional theory calculation displayed that Cr3+ increased ENR removal as a bridge ligand. Specifically, Cr3+ formed stable bidentate with carboxylic group and monodentate complex with pyridinic N on the material surface. The central Cr3+ further established coordination linkages with O containing groups of ENR. This study systematically investigates the interaction mechanisms between Cr(VI) and different antibiotics.