Coupling coagulation process based on flocs recycling: the key role of flocs interface regulation and synergistic...

Effective removal of emerging organic pollutants during coagulation is often limited by the weak affinity between the coagulant and the target pollutant. Here, with a focus on resource reuse, a recycled composite aluminum-iron-humic acid floc (CAFM-HA floc) was introduced, establishing a synergistic system with a covalent aluminum coagulant (CAS) to achieve enhanced coagulation of ciprofloxacin (CIP). The results showed that the recycled flocs formed under slightly acidic conditions (pH = 6) exhibited the highest interfacial reactivity, significantly increasing the removal efficiency to 71.8% compared to CAS alone (44.2%). Mechanistic analysis revealed interface adsorption of CIP onto the organic-rich flocs surface facilitated the formation of larger composite pollutants. The phenolic hydroxyl groups acted as hydrogen-bond donors, while the deprotonated carboxyl groups formed strong electrostatic interactions and ligand-like complexes with the protonated amino groups of CIP. Specifically, the introduction of flocs favored the dominant hydrolysis species of CAS, thereby improving its utilization efficiency. Overall, the presence of flocs shifted the system from a charge-neutralization-dominated to a multipath synergistic mechanism, effectively enhancing the affinity between CIP and coagulants. This study offers a new strategy to simultaneously achieve floc valorization and enhanced antibiotic removal.