Elsevier

Science of The Total Environment

Volume 651, Part 2, 15 February 2019, Pages 2845-2856
Science of The Total Environment

H2O2-assisted photoelectrocatalytic degradation of Mitoxantrone using CuO nanostructured films: Identification of by-products and toxicity

https://doi.org/10.1016/j.scitotenv.2018.10.173Get rights and content

Highlights

  • Needle-like CuO nanostructures were grown on Si substrates using a CBD method.

  • High photoelectrocatalytic performance of CuO film is related to its needle-like nanostructure.

  • Enhanced H2O2-assisted PEC degradation by CuO film electrodes it was demonstrated.

  • The MTX drug exhibits high acute toxicity against Artemia salina and Allium cepa.

  • By-products generated by photoelectrocatalysis are substantially less toxic.

Abstract

CuO nanostructured thin films supported on silicon with 6.5 cm2 area (geometric area greater than the studies reported in the literature) were synthesized by a chemical bath deposition technique. The electrodes were characterized by MEV, XRD, XPS, contact angle, cyclic voltammetry and electrochemical impedance spectroscopy analyses. To evaluate the photoelectrochemical properties of the CuO films, photocurrent–voltage measurements were performed using linear voltammetry. The catalytic activities of CuO nanostructures were evaluated by monitoring photodegradation of Mitoxantrone (MTX) under UV-A light irradiation. The method of photoelectrocatalysis (PEC), applying a voltage of 1.5 V and assisted by adding H2O2, was undertaken. To the best of our knowledge, no studies on the degradation of anticancer agents using PEC process have been found in the literature. For comparison purposes, experiments were performed under the same conditions by assisted photocatalysis (PC) with H2O2 and direct photolysis. CuO deposits consist of a needle-like morphology. The presence of CuO in the tenorite phase was evidenced by XRD and the XPS spectra showed the presence of copper(II) oxide. The increase in current under illumination shows that CuO exhibits photoactivity. The PEC system showed a 75% level of MTX degradation, while the level achieved using PC was 50%. Under UV-A light alone only 3% removal was obtained after 180 min. Up to 10 by-products were identified by chromatography-mass spectrometry (LC-MS) with m/z values ranging between 521 and 285 and a plausible degradation route has been proposed. It is worth mentioning that 9 by-products identified in this work, were not found in the literature in other studies of degradation or products generated as metabolites. The toxicity tests of MTX before and after PEC treatment with Artemia Salina and Allium cepa showed a decrease in the acute toxicity of the medium as the antineoplastic was degraded.

Introduction

The pollution of water bodies by chemical contaminants is one of the greatest problems the world is facing today, increasing with every passing year and causing serious and irreparable damage to the earth and population. Globally, about 1 billion people have no access to drink water supply and around 2.6 billion have no basic sanitation (Ibhadon and Fitzpatrick, 2013). Consequently, urban centers are seeking alternative sources of water supply that may supplement variable rainfall and the demand for population growth, and one of the options considered is the supply of drinking water with recycled water after advanced treatment (Rodriguez et al., 2009).

However, various organic compounds such as pharmaceuticals, personal care products, pesticides, flame-retardants, household chemicals and industrial chemicals pose a huge barrier to these applications, since they are resistance to conventional chemical, biological and photolytic processes (Garcia-Segura and Brillas, 2017). As a result, they have been detected in several aquatic environments at concentrations ranging from nanograms to micrograms per liter (Knopp et al., 2016).

The existence of pharmaceuticals in aquatic environments have been receiving increasing attention in the scientific community as potential micropollutants because they are not naturally biodegradable and show “pseudo persistence”, consequently, being found in surface water, groundwater, urban wastewater and also drinking water (Kanakaraju et al., 2018).

Of particular concern among the pharmaceuticals are residues from anticancer agentes (substances that are used to treat cancer) due to their mutagenic, carcinogenic, and genotoxic potential, even at trace levels.

Mitoxantrone or (1,4-dihydroxy-5,8-bis[[2-(2-hydroxyethyl)-amino]-ethyl]-amino]-anthraquinone dihydrochloride; MTX) is a synthetic anthracenedione anti-tumor drug and has been extensively used for the treatment of various malignancies such as advanced breast and prostate cancer, lymphoma and leukaemia (Rossato et al., 2013).

MTX is is eliminated from the body in about 6–11% by urine and 25% in feces and the remainder in the metabolized form (Gómez-Canela et al., 2015). Cytostatic drugs and human metabolites are most often directly discharged into the sewage system, without any specific control after being administered in hospitals (Zhang et al., 2013a). In addition, urban wastewaters receive a substantial contribution of excreted anticancer drugs as the result of outpatient treatment (Isidori et al., 2016).

Conventional treatment is not efficient in removing micropollutants (Cristale et al., 2016), including cytostatic drugs and their metabolites, because these compounds are not readily biodegradable, and they may become persistent in sewage sludge and consequently can be released into surface water from effluents (Isidori et al., 2016). It is therefore extremely important to research and develop technologies that are capable of eliminating these harmful compounds.

Heterogeneous catalysis in conjunction with advanced oxidation processes such as semiconductor photocatalysis has emerged as a promising technique for degrading refractory organics (Eswar et al., 2018). Photocatalysis (PC) requires nanomaterials to be excited and produce reactive species such as holes (hVB+), hydroxyl (HOradical dot), superoxide (O2radical dot) and perhydroxyl (HO2radical dot) radicals that are the main agents responsible for the degradation of organic contaminants (Banerjee et al., 2014; Eswar et al., 2018). However, e/h+ recombinations, which severely limit the achievement of high quantum yields, is the main problem that has to be addressed (Chong et al., 2010).

The application of electrochemical advanced oxidation processes (EAOPs) is an alternative solution for the effective removal of refractory emerging contaminants from water (Martínez-Huitle and Panizza, 2018). The environmental application of EAOPs has been the topic of several books and reviews, among the recent publications on these technologies, highlights one review article described by Sirés et al., 2014 and another presented by Brillas and Martínez-Huitle, 2015. Both reviews point to a broad discussion about the concepts related to these technologies and show advantages of their use for the remediation of pollution problems because the electron is a clean reagent.

The modification of photocatalysis through combination with electrochemistry in the so-called photoelectrocatalysis (PEC) method has been a promising alternative capable of minimizing the difficulties posed by PC, such as recombination of e/h+ pairs and post-treatment removal of catalysts (Sirés et al., 2014; Garcia-Segura and Brillas, 2017).

PEC is based on the application of a bias potential in a semiconductor photoanode simultaneously irradiated with photons of energy equal or greater than its band gap (Garcia-Segura and Brillas, 2017) and results ins significant increases in the degradation rates due to the synergy between electrocatalysis and PC (Eswar et al., 2018).

TiO2 is the most commonly used semiconductor in photocatalytic reactions (Ramos et al., 2015; Cavalcante et al., 2016). However, the photocatalytic activity of TiO2 is limited to UV region (wavelength < 390 nm, only about 3–5% of the solar energy) due to its wide band gap, around 3.2 eV for anatase TiO2 and 3.0 eV for rutile TiO2. Consequently, this increases operating costs and thus limits the practical applications of TiO2 in this technology (Banerjee et al., 2014).

In this context, cupric oxide, CuO, is an alternative to TiO2 because it can absorb effectively throughout the visible light region due to its direct band gap in the range 1.2–1.5 eV (Sreeju et al., 2017). Other characteristics that render CuO attractive as a photocatalyst include high solar absorbance, high optical absorption, high catalytic activity, low thermal emittance, good electrical properties, chemical stability, low toxicity, as well as being readily available and easy to synthesise by a number of different methods (Zhang et al., 2014).

Although many researchers have reported the application of different nanostructures of CuO in PC, such as, Li et al. (2011), Kim et al. (2015), Sreeju et al. (2017), the focus of these studies were the removal of dyes, which are considered as model molecules.

Limited information is available on the efficiency of this catalyst to explore the degradation of more complex molecules, such as pharmaceuticals. To the best of our knowledge, only two previous works studied the application of CuO for drug degradation by photocatalysis using CuO as semiconductor. Liang et al. (2017) synthesized Pt nanoparticles covered CuO nanosheet-like structures (Pt/CuO composite) and investigated their photocatalytic activities in the degradation of chlortetracycline (one of the components in tetracycline antibiotic) under visible light irradiation. Ahmadi et al. (2017) investigated the removal efficiency of the antibiotic ciprofloxacin using CuO nanoparticles. It is worth mentioning that these studies were performed by photocatalysis.

In contrast, only one study was found in the literature using CuO in the absence of TiO2 junction for PEC. Eswar et al. (2018) evaluated the performance of network structured CuO for removal of tetracycline antibiotic by PC and PEC. In this study, the authors used fluorine doped tin oxide glass slide, platinum foil and calomel were used as working, conter and reference electrodes, respectively in the presence of CuO nanoparticles. The results showed that PEC exhibited a threefold higher rate of antibiotic degradation compared to PC. In this context, limited information has been found in the literature for the use of CuO as working electrode for application in PEC. Consequently, this study is of great importance to contribute to the lack of studies aiming at the application of CuO electrodes as an alternative for replacing the electrodes based on TiO2, which are well known in the literature (Fu et al., 2009; Su et al., 2016).

In the present paper, we report the synthesis and detailed characterization of CuO nanostructured films and their application as an efficient photocatalyst for the PEC degradation of the emerging contaminant MTX. In addition, the intermediates formed during the PEC degradation were identified and reaction pathways have been proposed. Finally, acute ecotoxicity of the degradation products was evaluated against larvae of microcrustacean Artemia salina and Allium cepa (onion). This is the first study exploring the degradation of cytostatics drugs througouth PEC includindg also the investigation of the toxicity of the by-products generated from MTX degradation.

Section snippets

Materials

Silicon wafers with (100) surface orientation were purchased from Wacker-Chemitronic GMBH. Copper(II) chloride dehydrate (≥99,00%) was purchased from Sigma-Aldrich. Ammonia solution (28–30%) was obtained from Merck. MTX (98.01% pure) was purchased from Quiral Química do Brasil. The other reagents and solvents (purchased elsewhere) were used as received.

Synthesis of CuO films

CuO thin films on Silicon (Si) were synthesized by a chemical bath deposition technique based on the reports of Bayansal et al. (2012) and

Characterization results

The morphology of the films was investigated by SEM and the resultant images are shown in Fig. 2. It is found that the CuO deposits consist of a needle-like morphology and the nanoneedles have evenly covered the whole region of the Si substrate (geometric area ~6.5 cm2).

The average thickness value of the middle parts of the needle-like nanostructures were 47 ± 12 nm and the average length was 0.48 ± 0.13 μm, which is smaller than some sizes reported in the literature (Bayansal et al., 2012).

Conclusions

Needle-like CuO nanostructures on silicon with a geometrical area of ~6.5 cm2 have been successfully synthesized by a chemical bath deposition method using copper chloride as a starting material. It is worth mentioning that the area of the electrode prepared in this study is larger than the area of other electrodes reported in the literature. Detailed characterization confirmed the uniformity in both coverage and morphology of the CuO nanoneedles, which are crystalline and have a monoclinic

Acknowledgments

The authors thank the Brazilian funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes, Finance Code 001), Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul as well as the Irish funding agency Science Foundation Ireland (SFI) under the SFI PI programme (Grant No. 13/IA/1955).

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