Sunlight photodegradation of marbofloxacin and enrofloxacin adsorbed on clay minerals

• Solid-state photochemistry of fluoroquinolones (FQs) adsorbed on clays is studied.
• Sunlight-induced photodegradation profiles over irradiation time are reported.
• Photodegradation of FQs is supported also by X-ray diffraction data.
• Photoproducts and photochemical pathways are identified and commented.

Fluoroquinolone antibiotics (FQs) are important “emerging” micropollutants, and their environmental diffusion is an issue of great concern. In this study, the photochemical degradation of marbofloxacin (MAR) and enrofloxacin (ENR) adsorbed on montmorillonite (MMT) and kaolinite (KAO) clays was investigated. Being FQs photosensitive molecules, the sunlight-induced degradation of clay-adsorbed FQs and of their photoproducts was monitored as function of irradiation time. The photoproducts were identified by high performance liquid chromatography electrospray tandem mass spectrometry (HPLC-ESI-MS/MS) and the photochemical pathways have been elucidated. X-ray diffraction (XRD) has been employed to deeply study the solid-state photodegradation process of MAR and ENR on MMT. Interestingly, the XRD results clearly evidenced a significative variation of the interlayer spacing of MMT not only as function of FQs adsorbed amounts, but also in relationship with their photodegradation, not investigated by means of XRD as yet. The results confirmed that FQs adsorption is a bulk phenomenon that occurs in the interlayer spacing of the MMT structure; moreover, it was proved that sunlight largely degraded the adsorbed FQs, both on the external surface and in the interlayer spacing.