Drugs degrading photocatalytically: Kinetics and mechanisms of ofloxacin and atenolol removal on titania suspensions
Introduction
During the past fifteen years, pharmaceuticals and personal care products (PPCPs) have been recognized as an important class of organic pollutants due to their potential hazardous effects on humans and the aquatic ecosystem (Calza et al., 2008). Many studies have reported the presence of PPCPs at concentrations ranging between μg/L and ng/L levels in aquatic environments worldwide (Fatta et al., 2007). Pharmaceuticals are designed to have physiological effect on humans and animals at trace concentrations. These compounds are persistent against biological degradation and natural attenuation and, therefore, may remain in the environment for a long time (Klavarioti et al., 2009).
Antibiotics have been used for several decades in both human and veterinary medicine. They are often partially metabolized in the organism and are excreted in the form of the parent substance or as metabolites in urine into wastewater. These substances were shown to be quite resistant to biodegradation (Kümmerer, 2009). Ofloxacin is one of the most frequently used fluorinated quinolone-type antibiotics with a broad spectrum of activity against both Gram-positive and Gram-negative bacteria (Zivanovic et al., 2006).
Another important class of pharmaceuticals is β-blockers, which are also released in the environment through urban wastewater treatment plants' discharges (Gros et al., 2006). Atenolol is one of the most frequently used β-blockers against cardiovascular diseases, because of its anti-hypertensic and anti-arrhytmic properties (Arvand et al., 2008).
Significant concentrations of these pharmaceuticals have been detected in municipal sewage. According to Radjenović et al. (2009) a range of 0.09–31.7 μg/L was found for ofloxacin in the raw effluents of a Spanish wastewater treatment plant while the corresponding values for atenolol were 0.84–2.8 μg/L. The study also determined the daily aqueous mass output loads for the compounds though the treated wastewater which are 2.1–267.2 g/d for ofloxacin and 2.2–50.8 g/d for atenolol.
Conventional sewage treatment plants are not able to degrade residues of these chemicals, and as a result they are introduced into the aquatic environment (Nikolaou et al., 2007). During the past years many investigations on chemical and biological technologies have been reported for the decomposition of organic pollutants in aqueous matrices. In this context, various advanced oxidation processes (AOPs) have been successfully employed for the degradation of a wide range of organic pollutants in water and wastewater (Parsons, 2004). Among the various AOPs, heterogeneous semiconductor photocatalysis using TiO2 as the photocatalyst has been found capable of achieving complete oxidation of the organic pollutants via hydroxyl radicals HO and/or valence band holes h+ generated when the semiconductor is exposed to UV irradiation (Fujishima et al., 2008). TiO2 is cheap, commercially available in various crystalline forms and particle characteristics, non-toxic and photochemically stable. Moreover, TiO2 photocatalysis works at ambient conditions and may be induced by solar irradiation (Malato et al., 2009).
The present work focuses on the degradation of ofloxacin and atenolol via TiO2 photocatalysis and provides information on the influence of different parameters, including TiO2 loading, initial substrate concentration, addition of hydrogen peroxide, pH of the aqueous solution and the water matrix, on the drug conversion and dissolved organic carbon reduction. Moreover, the acute and chronic toxicity of ofloxacin and atenolol to D. magna prior to and after photocatalytic treatment is assessed. The original aspects of the present work include the elucidation of the kinetics and mechanisms of the ofloxacin and atenolol removal on titania suspensions and also the investigation of the toxicity potential of the solutions during TiO2 photocatalysis. In addition, to the authors' knowledge, this is the first study that includes a systematic examination of the various parameters that affect the oxidation process of the two pharmaceutical compounds including the type and loading of the catalyst, initial substrate concentration, addition of hydrogen peroxide, pH and water matrix.
Section snippets
Chemicals
Ofloxacin and atenolol (their chemical structures are shown in Scheme 1 and major properties are summarized in Table 1) were purchased from Sigma–Aldrich and used as received. Solutions containing each compound under investigation at concentrations up to 20 mg/L were prepared by adding the appropriate mass of the pharmaceutical to ultrapure water (UPW) and stirred for several hours to ensure complete dissolution. Such concentrations, although considerably greater than those typically found in
Results and discussion
Preliminary dark adsorption experiments were conducted to assess the extent of 10 mg/L substrate adsorption onto Degussa P25 surface at catalyst loadings ranging from 50 to 800 mg/L. Experiments were run for several hours but it was found that equilibrium could be reached in 30 min. In all cases, the extent of adsorption of atenolol and ofloxacin did not exceed 10% and 30%, respectively.
In further experiments, the extent of photolytic degradation was studied in the absence of catalyst, i.e. under
Conclusions
The degradation of ofloxacin and atenolol an antibiotic and a β-blocker, respectively was studied by means of UVA/TiO2 photocatalysis. The study included the investigation of the effect of various parameters such as type and loading of the catalyst, initial substrate concentration, addition of hydrogen peroxide, pH and water matrix. In addition the work entailed an assessment of the toxicity potential of the oxidized solutions during the photocatalytic process. The main conclusions drawn from
Acknowledgments
This work was funded by the Cyprus Research Promotion Foundation through grant AEIFO/0506/16 (project title: PHAREM).
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