Intrinsic mechanism for the removal of antibiotic pollution by a dual coagulation process from the perspective of the interaction between NOM and antibiotic

Antibiotics bring potential risks to human health and ecosystem, and their coexistence with natural organic matters (NOMs) could have harmful impacts on the environment. Herein, a polyaluminium chloride (PAC)-polydimethyl diallyl ammonium chloride (PDMDAAC) dual coagulation process was designed to remove the co-pollutants of chlortetracycline (CTC) and humic acid (HA), representing antibiotics and NOMs, respectively. The main research strength was given to understand molecular interactions and their mechanisms associated with the coagulation and flocculation. We found that the co-existing HA and CTC increased the hydrophily and stability of contaminants, and generated HA@CTC complexes with large particles size. The interaction mechanism between CTC and HA was mainly hydrogen bonding, hydrophobic association action, n-π* electron donor-acceptor interaction, and π-π* conjugation. Lewis acid-base interaction was the main force between HA and CTC. The bonding energies of OH…N, OH…O, and hydrophobic association were -12.2 kcal/mol, -13.1 kcal/mol, and -11.4 kcal/mol, respectively, indicating that hydrogen bonding was stronger than hydrophobic association. The interactions between HA and CTC could improve their removal efficiency in the coagulation process. This is due to that the functional groups (COOH and OH) in the HA@CTC could be adsorbed by Al based hydrolysates. Polar interaction dominated the CTC and HA removal, and PAC was more efficient than PDMDAAC to remove HA@CTC complexes due to its higher complexing capacity. Thanks to the low concentration of residual contaminants and the formation of large and loose flocs, the interaction of HA and CTC could alleviate membrane fouling during ultrafiltration process. This study will provide new insight into the efficient removal of combined pollution and membrane fouling control.