In Situ ATR-FTIR Analysis of Nalidixic Acid Degradation upon Oxygenation of Fe(II)-Sorbed Ferrihydrite and Maghemite Surfaces

The mechanisms of the abiotic degradation of antibiotics in redox-dynamic environments, such as aquifer sediments or hyporheic zones, remain poorly understood. This study investigates the transformation of nalidixic acid (NAL), a model quinolone antibiotic, triggered by the dark oxygenation of Fe(II) sorbed onto two (oxyhydr)oxides: nanomaghemite and ferrihydrite. Using in situ ATR-FTIR spectroscopy under continuous flow, we demonstrate that exposing the Fe(II)-sorbed surfaces to dissolved O2 initiates the oxidation of adsorbed NAL over timescales of hours, without requiring light or added strong oxidants as H2O2. Real-time spectral analysis reveals mineral-dependent fates of transformation products. On Fe(II)-sorbed maghemite, significant vibrational shifts indicate the oxidative cleavage of the pyridinic ring, mirroring mechanisms observed in heterogeneous Fenton systems, leading to the accumulation of adsorbed byproducts. Significant degradation of NAL is also observed upon oxygenation of Fe(II)-sorbed ferrihydrite, with a slightly different end-product signal. These findings highlight that the aeration of Fe(II)-bearing minerals is a potent natural pathway driving the deep structural transformation of recalcitrant contaminants during environmental redox cycles.