Adsorption of Levofloxacin onto Chitosan- and Layered Double Hydroxide-Functionalized Montmorillonite

Antibiotic contamination of water resources is a major environmental concern. Beyond their direct toxicity to nontarget aquatic organisms, antibiotic-contaminated waters can serve as reservoirs for the development of antimicrobial resistance (AMR). Levofloxacin (LVX), a third-generation fluoroquinolone antibiotic, has frequently been detected in various environmental media. In this study, the LVX adsorption performance of chitosan/montmorillonite (CTS/MMT) and CO₃-type MgAl layered double hydroxide/montmorillonite (LDH/MMT) composites was evaluated in batch experiments. MMT, a naturally occurring phyllosilicate clay, was functionalized with either the polysaccharide CTS or the anionic clay LDH. The synthesized CTS/MMT composite maintained a negative surface charge across the whole pH range from 2.0 to 12.0, whereas LDH/MMT exhibited an isoelectric point (IEP) at pH 5.1. Key operational parameters, including solution pH, contact time, temperature, and initial LVX concentration, were then optimized. Both adsorbents showed maximum adsorption at pH 3 in a 30 mg L^-1 LVX solution after 60 min, with removal efficiencies of 77.7% and 99.1% for 0.01 g of LDH/MMT and CTS/MMT, respectively. Temperature variation data showed that LVX adsorption efficiency for LDH/MMT remained largely unaffected, ∼78.0% for a 0.01 g dosage, ∼90.0% for 0.05 g, and ∼87.0% for 0.1 g throughout the temperature range 15-35 °C. In contrast, CTS/MMT consistently achieved near-complete LVX removal (∼100.0%) across all tested temperatures regardless of dosage. For 0.01 g of adsorbent dosage, CTS/MMT adsorbed >99.0% LVX within 60 min, whereas adsorption onto LDH/MMT achieved equilibrium within 10 min with 77.1% removal efficiency. The Langmuir model fitted the CTS/MMT adsorption data well, with a maximum adsorption capacity of 107.53 mg g^-1, whereas the Freundlich model better described the adsorption behavior of LDH/MMT. For CTS/MMT, at pH < 5.59, surface adsorption through electrostatic attraction between cationic LVX and the negatively charged composite surface, coupled with cation exchange into the MMT interlayer, may be the principal adsorption phenomenon. On the other hand, heterogeneous intercalation of cationic LVX molecules into the MMT interlayer was the principal mechanism for LVX adsorption onto LDH/MMT.