Solar photo-Fenton degradation of nalidixic acid in waters and wastewaters of different composition. Analytical assessment by LC–TOF-MS

This work assessed the solar photo-Fenton degradation of nalidixic acid (NXA), a quinolone antibacterial agent, in several different aqueous solutions. It has been proven that the composition of the water clearly affects the efficiency of the photo-Fenton process. The presence of chlorine ions induces the concurrence of different mechanisms involving Cl and Cl2− radicals, which slow down the process. Up to 35 transformation products (TPs) were identified and their structures characterized by accurate LC–TOF-MS mass measurements during treatment of the different model waters. Photocatalytic degradation was thus observed to proceed mainly through the attack of the hydroxyl radicals on the double bond C(2)C(3) which induce further ring opening. All the TPs identified persisted after total degradation of NXA. NXA in real pharmaceutical effluent was treated by photo-Fenton as a first stage before biological treatment. As NXA has been demonstrated to be recalcitrant to biological treatment, photo-Fenton treatment of the effluent was continued until its total degradation. Although NXA was efficiently degraded, LC–MS analyses demonstrated that some of the TPs identified after the photo-Fenton treatment were also recalcitrant to biological treatment, persisting after the combined treatment. These results show that analytical assessment of photocatalytic water treatments is essential to assure they are functioning as intended.

Research highlights
► This work assessed the solar photo-Fenton degradation of nalidixic acid in aqueous solutions.
► The mineralization rate was slower as the more complex the matrix was.
► Up to 35 transformation products were identified by LC–TOF-MS.
► High concentrations of chloride revealed the formation of chlorinated by-products.
► Monitoring of transformation products becomes fundamental for optimization of water treatments.