Environmental photochemistry of Fluoroquinolones in soil and in aqueous soil suspensions under natural solar light

1. Introduction Fluoroquinolones (FQs) are a family of synthetic, broad-spectrum antibiotics as human and veterinary drugs. FQs are metabolized only in small part, therefore a large fraction of the ingested dose is excreted unmodified and reaches environmental waters in the pharmaceutically active form. These compounds rapidly move to solid matrices, i.e. sediments and soil, due to the high distribution coefficients [1], and eventually undergo photodegradation. Degradation rates and, consequently, persistence depend on the ambient conditions. Photochemical processes are largely affected by matrix composition, i.e. on the physical state of the drug (in solution or adsorbed on soil particles), on light power and on the role of inorganics and natural organic matter acting as photosensitizers [2-4]. FQs are emerging contaminants that have been detected worldwide in natural waters [5] and soils [6] at concentrations at the ppb level and below. With the aim to explore the FQ environmental fate in such matrices, we have studied their photodegradation in natural water [6-8] and in sunlight-exposed soil [9]. Photochemistry plays a major role in the environmental fate of FQ pharmaceuticals, and the study of the resulting photoproducts is of great importance in order to avoid underestimating the FQ environmental impact [8, 10, 11]. 2. Results The photodegradation fate of widely used Fluoroquinolones (FQs) in solid environmental matrices has been studied at actual concentrations (500 ng g-1) under natural solar light. Both human and veterinary drugs have been chosen, namely Ciprofloxacin (CIP), Danofloxacin (DAN), Enrofloxacin (ENR), Levofloxacin (LEV), Marbofloxacin (MAR) and Moxifloxacin (MOX). In this work, the photochemical behaviour of adsorbed FQs at the water-soil interface was investigated for the first time, in view of the great relevance to natural ecosystems and compared to their photodegradation in sunlit soil. After spiking and irradiation, samples were submitted to microwave-assisted extraction and analyzed by high performance liquid chromatography coupled to fluorescence detection (HPLC-FD). The photochemical fate of the parent compound and the time evolution of byproducts was monitored at the ppb levels by HPLC-FD. This proved that FQs are degraded considerably in these matrices. The degradation rates measured in aqueous soil suspension were higher than in neat soil, but lower than in water solution. A number of different byproducts, identified by HPLC electrospray ionization tandem mass spectrometry ESI-MS/MS after a post-extraction cleanup based on a molecularly imprinted polymer-phase, were formed during the photolytic process. The analysis of the photoproducts shows that on solid matrix (viz. FQs adsorbed on soils) the main photoprocess is the oxidative degradation started from the most labile moiety, the amine side-chain. On the contrary, the two main photodegradative paths observed in solution for this class of molecules, viz. dehalogenation and photosubstitution, are absent or remain only as secondary processes in the case of the less reactive FQs (LEV, MOX). The long time required for the photodegradation and the low reactivity make the two latter paths competitive with degradative oxidation, which however remains the predominant path. When a borderline situation is explored, viz. aqueous soil suspensions, our results suggest that the primary photochemical processes are due to soil adsorbed rather than to water dissolved molecules, further underling the strong interaction of FQs with the soil matrix. 3. Conclusions We have explored the environmental photochemistry of six widely used FQs in two important matrices, specifically agricultural soil and aqueous soil suspension, the latter not previously studied. Natural solar light proved to degrade consistently such drugs (nanograms per gram level), with higher degradation rates found in aqueous soil suspension than in neat soil. In particular, among the FQs studied, LEV and MOX were the two more persistent drugs. The results we obtained indicate that the persistence of these antibiotic compounds is longer in such matrices than in surface water, that is in soil > aqueous soil suspension > water.