Graft-copolymerized chitosan induces spontaneous flotation-coagulation for rapid antibiotic removal: Performance and mechanism

UV 254 and turbidity removals reached 79.5% and 94.3%, respectively. • Maximum OFX removal reached 93.0% under optimized dosages. • DO-derived microbubbles were retained in three-dimensional floc networks. • Charge neutralization, bridging, adsorption, and flotation jointly drove removal. The persistent presence of antibiotics in aquatic environments, often coexisting with natural organic matter (NOM), poses potential ecological and health risks. In this study, a quaternized chitosan derivative (CTS-DAC) and polyaluminum chloride (PAC) were used to construct a rapid spontaneous flotation system for the removal of NOM and ofloxacin (OFX). CTS-DAC, prepared by grafting DAC onto chitosan, exhibited high charge density and a branched structure, thereby enhancing charge neutralization and adsorption bridging. Characterization of the flocs and coagulants indicated that the removal of NOM and OFX was governed by charge neutralization, adsorption bridging, sweep flocculation, and hydrogen bonding. Notably, NOM could associate with OFX, thereby facilitating antibiotic removal. At dosages of 2 mg/L PAC as Al 3+ and 6 mg/L CTS-DAC, the removal efficiencies reached 79.5% for UV 254 and 94.3% for turbidity. Density functional theory (DFT) calculations exclude strong O 2 chemisorption at hydroxyl-related sites, while electrostatic potential (ESP) analysis indicates enhanced surface polarity after CTS-DAC modification. Together with the DO-regulation experiments, these results suggest that the flotation behavior resulted from dissolved oxygen (DO) release and microbubble retention within the floc network. The highest OFX removal efficiency was obtained at dosages of 10 mg/L PAC as Al 3+ and 30 mg/L CTS-DAC, whereas stable spontaneous flotation was achieved within 20 min at PAC + CTS-DAC ratios of 1:1.5–1:2.5. This flotation time was substantially shorter than the separation time required in conventional coagulation systems (30–60 min). This work offers new insights into the efficient treatment of antibiotic-contaminated water and provides a strategy for optimizing coagulation processes. Download: Download high-res image (226KB) Download: Download full-size image