Transient Adsorption of Zwitterionic Fluoroquinolones on Goethite During Freeze–Thaw Cycles

Fluoroquinolones, a class of antibiotics, have been detected in various aquatic environments, including those experiencing freeze–thaw cycles. This study investigated the adsorption of ciprofloxacin (CIP) in frozen (−21 °C) and aqueous (25 °C) solutions under varying pH levels, electrolyte types, and ionic strengths. CIP sorption on goethite was found to be transient, as freezing re-establishes equilibrium, nearly doubling CIP loadings at acidic to circumneutral pH values. The original equilibrium was restored by thawing. Our investigation reveals that ion pairs, formed between the positively charged piperazine group of CIP and anions (Cl^–, Br^–, and NO₃^–), create a charge-shielding effect, explaining the transient nature of CIP sorption equilibrium at goethite-water interfaces. In situ ATR-FTIR observations and model predictions further confirm the significant role of ion-paired surface complexes in transient CIP sorption. The transience of CIP sorption equilibrium in frozen and aqueous solutions is attributed to the local concentrations of anions, which undergo freeze-concentration into liquid intergrain boundaries and dilution by reversible ice nucleation and thawing. As the interaction between the hydrosphere and cryosphere intensifies with climate change, these findings have significant implications for evaluating the fate of contaminants in both terrestrial and aquatic environments.