Surface Catalytic Degradation of Ciprofloxacin by Ferrihydrite Sulfidation Under Ambient Conditions

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•), primarily produced from the reaction between O and surface-bound Fe(II), was the dominant contributor to the accelerated transformation of CIP. Upon being attacked by ROS, CIP underwent degradation via carboxyl substitution, defluorination, hydroxylation, piperazine ring ketonization and piperazine ring cleavage. Additionally, common water quality factors, i.e., pH, natural organic matter (NOM) and inorganic anions could also significantly influence the degradation processes of CIP during ferrihydrite sulfidation under ambient conditions. This research offers valuable insights into the significant function of mineral sulfidation in facilitating the formation and reactivity of ROS, which sheds light on enhanced elimination of organic contaminants in sediments.