Degradation of fifteen emerging contaminants at μg L−1 initial concentrations by mild solar photo-Fenton in MWTP effluents
Introduction
Due to their growing use, pharmaceuticals, like the anti-inflammatory ibuprofen, the antibiotic flumequine and the antiepileptic carbamazepine, endocrine disruptors like bisphenol A and atrazine, personal care products like oxybenzone and parabens (PHBA), synthetic musks and fragrances like musk xylene and galaxolide, pesticides like isoproturon and endosulphan and illicit drugs like THC aznd cocaine, to name just a few, and other xenobiotic substances, are found in increasing quantities in wastewater, surface water, and even in drinking water (Kasprzyk-Hordern et al., 2009, Kim et al., 2007, Mitch et al., 2003, Esplugas et al., 2007, Ternes, 1998). Since the use of these substances cannot be controlled or eliminated as they are ever present in our daily lives, their release into the environment has to be optimized and restricted, as they pose risks to the environment, public health and aquatic systems and they are responsible for building up microbiological resistance, feminisation of higher organisms and ecotoxicological issues (Laville et al., 2004). Particularly relevant examples of such emerging contaminants (ECs), such as those mentioned above, which are ubiquitously present in influents and effluents of MWTPs in the high ng L−1 to low μg L−1 range, do not need to be persistent to be hazardous, because they are introduced continuously into the environment (Fono et al., 2006, Jackson and Sutton, 2008, Nakada et al., 2008, Petrovic et al., 2003).
Conventional MWTPs, typically based on biological processes, are capable of removing some substances, but non-biodegradable compounds may escape the treatment and be released into the environment (Göbel et al., 2007, Carballa et al., 2004, Ternes et al., 2007). ECs have been found in the MWTP effluents at mean concentrations ranging from 0.1 to 20 μg L−1 (Castiglioni et al., 2006, Martínez Bueno et al., 2007, Radjenović et al., 2007, Richardson, 2007, Zhao et al., 2009). Concern about the growing problem of the continuously rising concentrations of these compounds must be emphasized, and therefore, the application of more thorough wastewater treatment protocols, including the use of new and improved technologies, is a necessary task.
Reusable water should be free of these persistent, toxic, endocrine-disrupting or non-biodegradable substances, (Radjenović et al., 2007, Teske and Arnold, 2008), and therefore, an effective tertiary treatment is required to remove these substances completely.
Among the advanced technologies that may be used to remove these pollutants, (Gogate and Pandit, 2004, Saritha et al., 2007, Huber et al., 2005) advanced oxidation processes (AOPs), through the generation of hydroxyl radicals which are able to mineralise most organic molecules yielding CO2 and inorganic ions as final products, are a particularly attractive option. (Farré et al., 2005, Gebhardt and Schröder, 2007, Gültekin and Ince, 2007, Ning et al., 2007, Rosenfeldt and Linden, 2004, Rosenfeldt et al., 2007, Ternes et al., 2003) The generation of the OH radicals can be achieved: electrochemically (Cañizares et al., 1999, Pelegrini et al., 2001, Zhou et al., 2005), sonochemically (Mantzavinos et al., 2004, Papadaki et al., 2004, Lesko et al., 2006), photochemically (Esplugas et al., 2005, Bali et al., 2003, Bremner et al., 2006), and by homogeneous or heterogenous catalysis (Zepp et al., 1992, Martinez et al., 2005) in acid or basic media (Glaze et al., 1987, Hislop and Bolton, 1999, Neyens and Baeyens, 2003). Most of the AOPs make use of a combination of either oxidants and irradiation (O3/H2O2/UV), or a catalyst and irradiation (Fe2+/H2O2; UV/TiO2). The drawbacks which make them economically disadvantageous depend on the specific AOP: (i) High electricity demand (e.g. ozone and UV-based AOPs), (ii) the relatively large amounts of oxidants and/or catalysts (e.g. ozone, hydrogen peroxide and iron-based AOPs), and (iii) the pH operating conditions (e.g. Fenton and photo-Fenton). This is why, although AOPs are well known for their capacity for oxidising and mineralising almost any organic contaminant, commercial applications are still scarce. Processes like photo-Fenton may be applied to commercial applications by using solar energy as a light source, optimizing the pH range and the amounts of oxidant/catalyst required.
AOP efficiency in the removal of ECs has typically been studied in demineralised water and bench scale, at initial concentrations in the milligram-to-gram range, which is not realistic compared to the concentrations detected in real water and wastewater (Farré et al., 2005, Lapertot et al., 2007, Kassinos et al., 2009, Malato et al., 2007). This work focused on solar photo-Fenton degradation of the ECs typically found in the effluents of MWTPs, leaving the treated wastewater suitable for reuse. Moreover, to make the process of interest for practical applications high iron concentrations (mM range), excessive amounts of H2O2 and a pH under 3 must be avoided (Pignatello et al., 2006). A new approach aimed at finding a very mild photo-Fenton treatment (low iron concentration and H2O2 dose at neutral pH), has been proposed (Moncayo-Lasso et al., 2008, Klamerth et al., 2009). In this paper, a pilot-scale solar photo-Fenton treatment was run at a pH between 6 and 7, with starting concentrations of 5 mg L−1 Fe, and 50 mg L−1 H2O2. Synthetic water (SW), simulated effluent wastewater (SE) and real effluent wastewater (RE) were tested in this study to which a mixture of 15 ECs, consisting of pharmaceuticals, pesticides and personal care products, selected from a list of 80 compounds found in MWTP effluents in previous studies (Martínez Bueno et al., 2007), were added at low concentrations (100 μg L−1 each).
Section snippets
Reagents
All reagents used for chromatographic analyses, acetonitrile, methanol, and ultrapure (MilliQ) water, were HPLC grade. Analytical standards for chromatography analyses were purchased from Sigma-Aldrich. Table 1 and Scheme 1 list the 15 compounds (pharmaceuticals, pesticides and personal care products) used. Photo-Fenton experiments were performed using iron sulphate (FeSO4·7H2O), reagent-grade hydrogen peroxide (30% w/v), sulphuric acid and hydrochloric acid for carbonate stripping, all
Photo-Fenton tests using SW
Previous experiments have confirmed that low iron concentrations without pH adjustment can lead to extremely slow degradation kinetics because of the presence of carbonate species (CO32− and HCO3−) which compete with organic contaminants for hydroxyl radicals (Klamerth et al., 2009). Therefore, SW was acidified (41 mg L−1 HCl) to remove carbonates prior to photo-Fenton. 15 min recirculation in the pilot plant was necessary to remove carbonates (final TIC = 5 mg L−1) with pH between 5.2 and 6.0.
Conclusions
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The experiments showed that ECs at low concentrations (μg L−1 range) can be successfully degraded to negligible concentrations with solar photo-Fenton at low iron concentrations (5 mg L−1) and low initial H2O2 (50 mg L−1) concentrations without adjusting the pH.
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One limiting factor for the degradation rate is the presence of CO32− and HCO3- which are very efficient OH radical scavengers and which have to be removed prior to the photo-Fenton reaction. This can be done either with HCl or H2SO4, as the
Acknowledgements
Funding for this work was provided by the Spanish Ministry of Science and Innovation under the Consolider-Ingenio 2010 programme (Project CSD2006–00044 TRAGUA; http://www.consolider-tragua.com) and by the Andalusia Regional Government (Project no. P06-TEP-02329). Nick Klamerth would to thank the University of Almería and CIEMAT for his Ph. D. research grant. Luigi Rizzo wishes to thank the University of Salerno (Italy) and the Province of Salerno for the research grant which allowed him to work
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