Photocatalytic degradation of the antibiotic chloramphenicol and effluent toxicity effects
Introduction
Emerging contaminants are continually discharged into the aquatic environment without any restriction posing potential risks for public health and the environment. Antibiotics have been increasingly detected in sewage water, natural water, surface water and groundwater (Chatzitakis et al., 2008, Fatta-Kassinos et al., 2011, Van Doorslaer et al., 2015). Antibiotics are readily available on the market being used to treat diseases in humans and in animals, promote animal growth and improve nutritional efficiency of feed (Sarmah et al., 2006). Despite their low environmental concentrations (from ng L−1 to μg L−1), the continuous input and persistence into the aquatic ecosystem make antibiotics one of the most urgent environmental issue, primarily due to the potential for the development of antimicrobial resistance (Dunlop et al., 2015).
The limitations of conventional wastewater treatment plants (WWTPs) in removing these bio-recalcitrant molecules point toward the urgent need for improved wastewater treatments such as Advanced Oxidation Processes (AOPs), a special class of oxidation techniques characterized by production of •OH radicals. Amongst several AOPs, heterogeneous photocatalysis has proven its potential in degrading antibiotics from aqueous matrices (Zhang et al., 2010, Lofrano et al., 2014, Van Doorslaer et al., 2015). The elimination of mother compounds does not necessarily result in toxicity removal, since the photocatalytic degradation can produce intermediate by-products, which can still exert adverse biological effects. Therefore to evaluate the overall behavior and efficiency of the process, it is worth to assess not only the removal of a specific compound, but also of the whole ecotoxicity potential (Carotenuto et al., 2014, Libralato et al., 2010a, Libralato et al., 2016, Lofrano et al., 2014, Rizzo et al., 2009). So far, ecotoxicity data for AOPs treated solutions of antibiotics are scarce or missing, making their environmental risk assessment difficult.
In the present study, the photocatalytic degradation of chloramphenicol sodium succinate (CAP, C15H15Cl2N2 Na2O8), which is a representative antibiotic applied to inhibit Gram-positive and Gram-negative bacteria, was investigated at various aqueous suspensions of TiO2. Its photo-degradation by-products as well as the toxicity of the final treated effluent were assessed as well. CAP has been widely used due to its low cost and high efficiency in the treatment of various infectious diseases. Due to its carcinogenic effects and other serious adverse reactions, such as bone marrow depression, aplastic anemia and severe blood disorders, CAP has been banned from China, Japan, Canada, United States, Australia and European Union in animals used for human consumption, even if it is still legally used in Brazil and other countries, or illegally, in livestock, due to the easy access, low price and steady antibacterial effectiveness (Andrade et al., 2006). As consequence CAP has been found in concentrations between 0.001 and 0.031 μg L−1 in surface waters in Singapore and Korea, respectively. Average CAP concentrations between 2.08 and 26.6 μg L−1 were found in effluents of sewage treatment plants in China (Choi et al., 2008, Liu et al., 2009, Peng et al., 2006, Xu et al., 2011). The degradation of CAP has been evaluated by several AOPs such as UV/H2O2 (Baeza et al., 2007), photo-Fenton (Trovó et al., 2013), photoelectron Fenton (Garcia-Segura et al., 2014), photocatalysis (Chatzitakis et al., 2008, Zhang et al., 2010), electrochemical degradation (Rezende et al., 2010). None of these carried out ecotoxicity tests on the TiO2 photo-catalytically treated effluent. Data on CAP ecotoxicological effects are available only for single species like for Vibrio fischeri (EC50=20.68 mg L−1) (Choi et al., 2008) and Daphnia magna (EC50=1086 mg L−1 (Calleja et al., 1994); EC50=227–600 mg L−1 (Müller, 1982); EC50=542.86 mg L−1 (Lilius et al., 1994)). Currently, no toxicity data are available for widely used photosynthetic biological models like the microalga Pseudokirchneriella subcapitata and dicotyledonous macrophyte Lepidium sativum.
The aim of this study was to elucidate the photocatalytic degradation kinetics of CAP (increasing contact times and photocatalytic agent concentrations, i.e. TiO2) and to assess the efficiency of degradation processes through the removal of ecotoxicological effects related to the potential by-product residues applying the principles of the whole effluent assessment (WEA) (OSPAR Commission, 2007, Libralato et al., 2010b) also in order to meet the goal of the best available technology (BAT). A battery of acute (A) and chronic (C) toxicity tests was used including biological models belonging to various trophic levels like V. fischeri (A), P. subcapitata (C), L. sativum (A) and D. magna (A). The toxicity of CAP as a pure substance was investigated on P. subcapitata and L. sativum due to missing data.
Section snippets
Reagents and analytical procedures
All reagents were of analytical grade. Photocatalytic degradation experiments were carried out using gravimetrically measured aliquots of TiO2 Degussa P25. The decay of CAP dispersed in ultra-pure water was followed by HPLC-UV (Finnigan Surveyer) equipped with a reversed phase C18 analytical column (Vydac, 5 μm, 150 mm×3.0 mm). The injection volume was 10 μL and the wavelength set for the quantification was 275 nm according to the maximum light absorption of CAP. The limit of quantification (LOQ)
Kinetic studies
Experiments on CAP at 25 mg L−1 carried out under dark at various (0.1, 0.2, 0.4, 1.6, 3.2 g L−1) TiO2 concentrations proved that adsorption did not influence significantly the CAP degradation. As shown in Fig. 1A, the CAP plotted as function of time showed a very slight removal after 120 min as an effect of the potential adsorption.
As for most of the organic compounds, the photolysis of CAP results strongly influenced by both the wavelength and intensity of UV source. Previous studies proved that a
Conclusions
Photocatalysis mediated by TiO2 was very efficient to mineralize, degrade and detoxify water spiked with CAP. After 120 min photo-reaction in presence of 1.6 g TiO2 L−1, CAP and its by-products were completely removed. Results obtained by the adsorption in the dark and direct photolysis for 120 min proved that both adsorption and photochemical processes did not significantly influence the observed fast transformations when the solution of CAP was irradiated in presence of TiO2. According to all
Acknowledgments
Authors acknowledge the PON Aquasystem project (Gestione Integrata del Ciclo delle Acque finalizzata all’Uso Sostenibile delle Risorse, all’Ottimizzazione Energetica, al Monitoraggio e Controllo della Qualità dell’Acqua nei Sistemi Acquedottistici e nelle reti di Drenaggio Urbano) funded by PON R&C 2007–2013, Smart Cities and Communities and Social Innovation.
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