Photodegradation kinetics and transformation products of ketoprofen, diclofenac and atenolol in pure water and treated wastewater
Highlights
► Direct UV photolysis of 3 pharmaceuticals in pure and waste water was investigated. ► Ketoprofen has higher photodegradion kinetics, followed by diclofenac and atenolol. ► MP/UV photodegradation products were identified for the 3 compounds. ► Photodegradation pathways were proposed to explain the obtained products. ► The persistent photoproducts were identified for each compound.
Introduction
In the past decade, there has been a growing concern about the discharge of pharmaceutical active compounds (PhAC) from wastewater treatment plants (WWTPs) [1], [2]. These compounds are designed to have an impact on life and are often resistant to biological degradation in the secondary treatment of conventional WWTPs. Therefore, it is important to assess the fate of PhACs in the tertiary treatment step of WWTPs where physical/chemical methods are often used.
UV irradiation is often employed for disinfection of drinking water and municipal WWTP [3], and it has also been demonstrated to effectively reduce the concentration of recalcitrant organic compounds [4], [5]. However, this process can also generate photodegradation intermediates that are more recalcitrant or toxic than the parent compounds. Low pressure (LP) mercury lamps – that emit monochromatic light at 254 nm – are known by their high disinfection efficiency since they irradiate at a wavelength in the range of the maximum absorption of DNA (240–260 nm). Medium pressure (MP) mercury lamps that emit light over a wider range of wavelengths have been reported as an effective alternative to LP lamps (e.g. [6]) and may be preferred in compact treatment systems, since the UV intensity per lamp is higher than in LP systems [3].
The compounds selected for this study were widely used pharmaceutical compounds with different chemical structures: ketoprofen, diclofenac (non-steroidal anti-inflammatory drugs), and atenolol (a β-blocker). These compounds were found in relatively high concentrations (up to 21.6 μg L−1 [7], [8]) in the effluent of the secondary settler of municipal WWTPs, thus reaching a subsequent disinfection process. The photolysis of ketoprofen, diclofenac and atenolol has been previously investigated, mostly in distilled water and surface water matrices (e.g. [4], [9], [10], [11]). Baeza and Knappe [12] recently studied the kinetics of diclofenac using low pressure (LP) direct and indirect UV photolysis in ultrapure water, lake water and also in wastewater effluent and found that the degradation rate was similar across all matrices. Good LP/UV photodegradation of ketoprofen and diclofenac in treated wastewater was demonstrated by Kim et al. [13], although atenolol only showed medium removal. Also Rosario-Ortiz et al. [5] reported low atenolol removal in WWTP effluent through LP/UV photolysis. Nevertheless, very few studies reported the effect of treated wastewater on photodegradation using MP lamps.
The objective of this study was to investigate the degradation kinetics and extent of transformation of selected PhACs by UV radiation used for wastewater disinfection purposes (i.e. direct photolysis). UV photolysis can be strongly affected by the presence of other organic compounds (other PhACs or dissolved organic matter), or particulate matter. In the present study, the photodegradation kinetics of ketoprofen, diclofenac and atenolol were assessed in a reactor equipped with a MP lamp, in filtered and unfiltered treated wastewater, and compared to the results obtained in pure water. Moreover, the photodegradation products of each compound were identified and monitored along irradiation time. To the best of our knowledge, this is the first study identifying the transformation products of ketoprofen, diclofenac and atenolol with direct photolysis, which enabled proposing photodegradation pathways for each compound.
Section snippets
Reagents
The PhACs used in this study were atenolol, diclofenac and ketoprofen (Discovery CPR, Sigma–Aldrich, Portugal). Atrazine (Discovery CPR, Sigma–Aldrich, Portugal) was used for actinometry of the UV MP lamp. The mobile phases used in high performance liquid chromatography (HPLC) were acetonitrile (HPLC grade, Panreac, Portugal) and ultra pure water obtained from Milli-Q50 system water purification (Millipore, Bedford, USA), acidified with formic acid (analytical grade, Merck, Portugal). The
Low pressure and medium pressure UV photoliability
Direct photolysis can only take place when a compound is able to absorb light at the wavelengths to which it is exposed. A measure of this absorption capacity, or photoliability, is given by the decadic molar absorption coefficient (ɛ), defined as the probability that a compound will absorb light at a certain wavelength (λ). In this study, the decadic molar absorption coefficient determined for the LP/UV lamp (λ = 254 nm) for ketoprofen was in the same order of magnitude as the value obtained by
Conclusions
The UV photolysis of ketoprofen, diclofenac and atenolol was investigated and the following conclusions are taken from this study:
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Decadic molar absorption coefficient and quantum yield values indicated high photodegradation for ketoprofen, followed by diclofenac, and low photolysis for atenolol;
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Time- and fluence-based rate constants determined in pure water confirmed these kinetics, suggesting that ketoprofen, and to a certain extent diclofenac, can be adequately removed in MP/UV disinfection
Acknowledgments
COST Action 636 is thankfully acknowledged for financial support through a Short Term Scientific Mission, and Fundação para a Ciência e a Tecnologia (FCT, Portugal) for funding through projects PTDC/AMB/65702/2006, PEst-OE/EQB/LA0004/2011 and PEst-C/EQB/LA0006/2011, and grants SFRH/PROTEC/49449/2009 and SFRH/BPD/30800/2006.
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