Factors affecting the fate of ciprofloxacin in aquatic field systems

Ciprofloxacin (cipro) is a broad-spectrum antibiotic used in human and veterinary medicine that is readily transported into the environment via domestic wastewaters and through direct runoff. Although factors governing cipro fate are becoming understood, an integrated evaluation of disappearance mechanisms in aquatic systems has not been performed. Here we examined cipro disappearance rate in surface waters using both laboratory and field systems under different light, and dissolved (DOC) and particulate organic carbon (POC) conditions to determine when photodegradation versus adsorption dominates cipro fate. Initial laboratory experiments showed that cipro rapidly photodegraded (t(1/2) ∼ 1.5 h) with numerous photodegradation products being noted when POC levels were low. However, even moderate water column POC levels resulted in reduced photodegradation ( no breakdown products detected) and soluble cipro disappearance rates were accelerated. C-14-ciprofloxacin studies confirmed significant adsorption onto aquatic POC (KOC values of 13,900 to 20,500 L/kg at neutral pH). In contrast, a follow-up mesocosm-scale field study using low POC water showed that photodegradation could also dominate cipro fate. In conclusion, both adsorption and photodegradation strongly influence cipro fate in aquatic systems, although the dominant mechanism appears to depend upon the ambient POC level.