Dissolved sulfide (S(-II)) is abundant in sediments and capable of initiating the sulfidation reactions of iron-bearing minerals, in which the reaction mechanisms have been well documented. However, the impact of the S(-II)/Fe concentration ratio on reactive oxygen species (ROS) formation and the fate of co-existing contaminants upon iron-bearing minerals sulfidation under ambient conditions remains inadequately explored. Herein, the transformation of ciprofloxacin (CIP) by ferrihydrite sulfidation under ambient conditions was systematically investigated. Our findings indicated that the rate of CIP degradation initially increased with rising S(-II)/Fe concentration ratios, but subsequently decreased as the ratio continued to elevate. Evidence from electron paramagnetic resonance, molecular probing and quenching experiments revealed that the superoxide anion radical (O