Dual Role of Ferrihydrite in Enrofloxacin Sorption: From Enhanced Retention to Competitive Inhibition in Semi‐Arid Agricultural Soils

ABSTRACT This study systematically investigates how ferrihydrite (Fh) amendment influences enrofloxacin (ENR) sorption in northwest China's agricultural soils, addressing the critical gap in understanding Fh–antibiotic interactions under realistic field conditions. Our results reveal that Fh amendment exerts a conditional dual effect on ENR sorption in soils: Under acidic to neutral conditions (approximately pH 4–6) and low ionic strength, low Fh dosages enhance sorption, whereas under alkaline conditions (pH > 8) or at high Fh dosages (≥ 6%), sorption is inhibited, likely due to competitive sorption and site blocking. Maximum enhancement (8.24%) occurred with 4% Fh. Both pristine and Fh‐amended soils reached equilibrium within 2 h, with pseudo‐second‐order kinetics providing the best fit. Linear models effectively characterized the thermodynamic sorption process. Analysis of sorption affinity at different temperatures showed both low and high temperatures reduced promotional effect of Fh on ENR sorption in soils. Under acidic conditions, protonation of the hydroxyl functional groups of Fh alters surface charge, decreasing the ENR sorption capacity of soil. Fh exhibits optimal enhancement of ENR sorption in weakly alkaline soil conditions. Notably, Fh addition strengthens the sorption‐promoting effect of low‐concentration cations, primarily through cation‐bridging mechanisms. However, Fh reduces the NH 4+ ‐enhanced sorption effect due to weakened electrostatic sorption and intensified competitive sorption. The sorption‐enhancing capability of Fh originates from its abundant surface functional groups and pH‐dependent charge characteristics. These findings demonstrate that appropriate Fh addition can effectively improve ENR immobilization, offering insights for agricultural soil management strategies.