Photocatalytic degradation kinetics, mechanism and ecotoxicity assessment of tramadol metabolites in aqueous TiO2 suspensions

https://doi.org/10.1016/j.scitotenv.2015.12.088Get rights and content

Highlights

  • Photocatalytic degradation of N-desmethyl, N,N-bidesmethyl and N-oxide-tramadol was studied.

  • Degradation of all studied compounds was strongly inhibited in wastewater.

  • Hydroxylation and oxidation are the main degradation pathways.

  • HO are the main reactive radicals during the photocatalytic process.

  • A significant abatement of the overall toxicity was accomplished.

Abstract

This study investigated for the first time the photocatalytic degradation of three well-known transformation products (TPs) of pharmaceutical Tramadol, N-desmethyl-(N-DES), N,N-bidesmethyl (N,N-Bi-DES) and N-oxide-tramadol (N-OX-TRA) in two different aquatic matrices, ultrapure water and secondary treated wastewater, with high (10 mg L 1) and low (50 μg L 1) initial concentrations, respectively. Total disappearance of the parent compounds was attained in all experiments. For initial concentration of 10 mg L 1, the target compounds were degraded within 30–40 min and a mineralization degree of more than 80% was achieved after 240 min of irradiation, while the contained organic nitrogen was released mainly as NH4+ for N-DES, N,N-Bi-DES and NO3 for N-OX-TRA. The degradation rates of all the studied compounds were considerably decreased in the wastewater due to the presence of inorganic and organic constituents typically found in effluents and environmental matrices which may act as scavengers of the HO. The effect of pH (4, 6.7, 10) in the degradation rates was studied and for N-DES-TRA and N,N-Bi-DES-TRA, the optimum pH value was 6.7. In contrast, N-OX-TRA showed an increasing trend in the photocatalytic degradation kinetic in alkaline solutions (pH 10). The major transformation products were identified by high resolution accurate mass spectrometry coupled with liquid chromatography (HR-LC–MS). Scavenging experiments indicated for all studied compounds the important role of HO in the photocatalytic degradation pathways that included mainly hydroxylation and further oxidation of the parent compounds. In addition, Microtox bioassay (Vibrio fischeri) was employed for evaluating the ecotoxicity of photocatalytically treated solutions. Results clearly demonstrate the progressive decrease of the toxicity and the efficiency of the photocatalytic process in the detoxification of the irradiated solutions.

Introduction

The presence of pharmaceuticals and their metabolites in surface waters up to several μg L 1 is an emerging environmental issue raising many concerns about the effect that they might have on the aquatic life and potentially on humans, through drinking water supplies (Radjenović et al., 2012, Kasprzyk-Hordern et al., 2009, Jelic et al., 2011).

Pharmaceuticals used in medicine enter wastewater as excretion of either unchanged parent compounds or metabolites with urine or through the improper disposal of unused or expired pharmaceuticals products (Song et al., 2008). In wastewater treatment plants (WWTPs), the pharmaceuticals undergo partial elimination by either biological or chemical degradation. An incomplete removal modifies the chemical structure resulting in various transformation products (TPs) with unknown properties. On the other hand, pharmaceuticals may also pass unchanged through WWTPs and in this case end up in the receiving aquatic compartments (Quintana et al., 2005, Song et al., 2008, Stamatis and Konstantinou, 2013).

After entering the environment, pharmaceuticals may undergo various biotic and abiotic processes which modify their chemical structure transforming them into various products. Metabolites and TPs generated during the above mentioned processes, can usually be more hydrophilic compounds, which show higher mobility potential and in some cases higher toxicity than their parent compounds (Fatta-Kassinos et al., 2011, Jeon et al., 2013).

Thus, studies on the TPs of pharmaceuticals formed during environmental processes and treatment of water and wastewater and the determination of their toxicity and ecotoxicity are fundamental and a prerequisite for comprehensive protection of the environment (Fatta-Kassinos et al., 2011).

N-desmethyl-tramadol (N-DES), N,N-bidesmethyl-tramadol (N,N-Bi-DES) and N-oxide-tramadol (N-OX) with chemical structures depicted in Fig. 1 are the bioactive metabolites of Tramadol (TRA), a widely used blood opioid analgesic of the aminocyclohexanol type (Wu et al., 2002, Hakala et al., 2006). TRA has been detected at concentrations as high as 1000 ng L 1 in secondary effluents of WWTPs (Rùa-Gόmez and Püttmann, 2012, Hummel et al., 2006), whereas concentrations in the range of 100 ng L 1 have been detected in surface waters (Rùa-Gόmez and Püttmann, 2012). Regarding its photolysis in natural waters, TRA has been reported to undergo slow photodegradation (Bergheim et al., 2012, Rùa-Gόmez and Püttmann, 2013). Although N-DES, N,N-Bi-DES- and N-OX-TRA have not been included in monitoring-studies, they are well-known TPs of TRA by biological (Lintz et al., 1981, Jeon et al., 2013) and chemical reactions (Zimmermann et al., 2012, Radjenović et al., 2012). N-DES-, N,N-Bi-DES- and N-OX-TRA have been identified as major TPs during the treatment of TRA using different remediation technologies such as ozonation, oxidation by ferrate (Zimmermann et al., 2012) and electrochemical oxidation (Eversloh et al., 2015).

No reports are available on the human health risk of exposure to N-DES-, N,N-Bi-DES- and N-OX-TRA up to now. Moreover, no conclusive data have been reported about the persistence of the studied compounds in the environment. However, it has been demonstrated that the co-presence of pharmaceutical compounds and/or their metabolites/TPs in water samples, can exert significant increased toxic effects, even at concentrations at which the substances individually showed either no effect or only a very slight one (Cleuvers, 2004). In general, pharmaceutical compounds and their metabolites/TPs have been characterized as persistent substances with a considerable potency of bioaccumulation in the environment. Consequently, human health concerns have been raised, as surface water is used as a source of drinking water production all over the world (Carbonaro et al., 2013, de Jongh et al., 2012).

Heterogeneous photocatalysis, a representative advanced oxidation process (AOP), has received considerable attention for the removal of a wide range of micropollutants including pharmaceuticals and their metabolites (Klavarioti et al., 2009, Yang et al., 2010, Doll and Frimmel, 2004, Antonopoulou and Konstantinou, 2013). It has been proved to be an advantageous technology as it can be carried out under ambient conditions and lead to complete mineralization of organic pollutants (Konstantinou and Albanis, 2004, Yurdakal et al., 2007, Antonopoulou et al., 2013).

Despite the potential of AOPs in the protection of the environment, studies focused on metabolite/TP removal are scarce. In this sense, the photocatalytic transformation of three major metabolites/TPs N-DES-, N,N-Bi-DES- and N-OX-TRA, of a persistent and widely detected pharmaceutical such as Tramadol, is studied for the first time. Thus, the objectives of the present study were: (i) to evaluate the degradation of the studied compounds in ultrapure water and environmentally relevant systems (secondary treated wastewater using low initial concentrations); (ii) to assess the effect of pH on the degradation efficiency; (iii) to identify the TPs by using accurate mass HR-LC–MS; (iv) to assess the contribution of reactive species involved in the reaction mechanisms using different scavengers; (v) to study the degradation pathways followed and (vi) to assess the ecotoxicity during the photocatalytic treatment using Microtox bioassay (Vibrio fischeri).

Section snippets

Materials

High purity (99.9%) standards of N-DES-, N,N-Bi-DES- and N-OX-TRA, were purchased from LGC Standards (Germany). Potassium iodide (KI) and p-benzoquinone (p-BQ) were obtained from Sigma-Aldrich. Titanium dioxide Degussa P25 (anatase/rutile 70:30, surface area = 56 m2 g 1) was used as photocatalyst. Solvents of HPLC-grade (acetonitrile (ACN), isopropanol (i-PrOH), methanol (MeOH) and LC-grade water) were supplied by Merck (Darmstadt, Germany). Methanol (MeOH) of LC-MS grade was supplied by Fisher

Photocatalytic degradation kinetics

Experiments were carried out firstly in the absence of light or catalyst in order to evaluate the extent of adsorption or photolysis processes respectively, on the removal of the studied metabolites and the results are summarised in Fig. 1S. Low adsorption (about 5%) was observed for all the studied compounds in TiO2 particles after 240 min under continuous stirring in the dark at room temperature (pH = 6.7). On the other hand, photolysis under simulated solar light resulted in a decrease of about

Conclusions

The photocatalytic degradation kinetics of three well-known metabolites of Tramadol in aqueous solutions were investigated in detail. The results showed that the degradation rate followed the trend: N,N-Bi-DES- > N-DES- > N-OX-TRA in both UW and WW. In UW and initial concentration of 10 mg L 1, almost 90% disappearance of the compounds was achieved within 30–40 min of irradiation. After 240 min of photocatalytic treatment, high percentages (about 80%) of TOC were accomplished and the nitrogen was

Conflict of interest statement

The authors declare that there are no conflicts of interest.

Acknowledgements

This research has been co-financed by the European Union (European Social Fund—ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)—Research Funding Program: THALIS MIS 379409. Investing in knowledge society through the European Social Fund.

The authors would like to thank the Unit of Environmental, Organic and Biochemical high resolution analysis-orbitrap-LC-MS analysis of the University of Ioannina

References (39)

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