Degradation of emerging contaminants at low concentrations in MWTPs effluents with mild solar photo-Fenton and TiO2
Introduction
Spain has the highest water deficit in Europe, with water exploitation rates (annual consumption/resources) of over 20%, which is generating serious social concern. Approximately 65% of this consumption is for agriculture, 20% for industry and 15% is domestic. Although possible methods for mitigating the scarcity of water are widely discussed in the media (e.g., desalination and inter-basin water transfer), and although they may be cost-effective, they are not the best options due to high environmental costs, energy consumption and/or specific infrastructures. Therefore, economically competitive reuse of water is a necessary environmental alternative to avoid the aforementioned energy consumption and direct environmental impact. Spain is also one of the countries with the highest water reuse, but still in small quantities; only 5% of wastewater is reused, whilst its potential for water reuse is 1300 Mm3, one order of magnitude more than at present [1].
One of the main sources of water for potential reuse comes from municipal wastewater treatment plants (MWTPs). However, quality demands for reusable water [2], [3], [4] are that it do not contain any toxic, endocrine-disrupting compounds or nonbiodegradable substances, such as pesticides, pharmaceuticals, hormones, synthetic fragrances, and others which escape conventional wastewater treatment [5], [6]. Special attention has recently been given to what are called “emerging contaminants”, mostly unregulated compounds that may be candidates for future regulation depending on research on their potential effects on health, and monitoring data regarding their occurrence. Particularly relevant examples of such emerging compounds are those which do not need to persist in the environment to cause a detrimental effect, because their high transformation/removal rates are compensated by their continuous introduction into the environment [7]. Concern about the growing problem of the continuously rising concentrations of these compounds must be emphasized [8], [9], [10]. These hardly biodegradable products, which resist treatment by conventional sewage plants, have been found in their effluents at mean concentrations ranging from 0.1 to 20.0 μg L−1 [11], [12]. Consequently, the application of more exhaustive wastewater treatment protocols, including the use of new and improved technologies, is a necessary task.
Oxidation technologies [13], among them AOPs (Advanced Oxidation Processes), are recently considered an interesting option to solve this problem, mainly because of their versatility [14], [15]. AOPs, generally defined as oxidation processes generating hydroxyl radicals, are responsible for organic degradation due to their strong oxidising power. They are therefore able to oxidise and mineralise almost every organic molecule yielding CO2 and inorganic ions. Most of the systems classified as AOPs make use of a combination of either oxidants and irradiation (O3/H2O2/UV), or a catalyst and irradiation (Fe2+/H2O2; UV/TiO2). The common drawback of such systems is the high demand of electrical energy for devices such as ozonators, UV lamps, etc., very often making such treatments economically disadvantageous. This is why, although AOPs are well known for their capacity for oxidising and mineralising almost any organic contaminant, commercial applications are still scarce. Future applications of these processes could be improved through the use of catalysis and solar energy. Therefore, investigation is focusing increasingly [16] on the two AOPs, which can be powered by solar radiation, i.e., light with a wavelength over 300 nm, homogeneous catalysis by the photo-Fenton reaction and heterogeneous catalysis by UV/TiO2.
Photo-Fenton and TiO2 treatments have been reported as effective methods for eliminating these compounds [17], but their main drawback is their relatively high operating cost. As the concentrations of these substances in reusable MWTP water effluents are normally below 20 μg L−1, conventional treatment with TiO2 slurries (hundreds of mg L−1, or more) [18], high iron concentrations (mM range), excessive amounts of H2O2 and a pH around 3 [19] would be unnecessary. Therefore, a very mild treatment with photo-Fenton (at an extremely low iron concentration and H2O2 dose, and no pH adjustment) or with highly diluted TiO2 slurries could be of interest for study.
The purpose of this paper is therefore technical evaluation of mild solar TiO2 and photo-Fenton as tertiary treatments in MWTPs. The contaminated water tested in this paper was a mixture of emerging contaminants at low concentrations (such as pharmaceuticals, pesticides and personal care products) selected from 56 compounds found in MWTP effluents in previous studies [12]. This research, the first results of which are included in this paper, is part of an ambitious programme (Treatment and Reuse of Waste Waters for Sustainable Management “TRAGUA”, http://www.consolider-tragua.com) financed by the Spanish Government, and involving the participation of a multidisciplinary team of 24 research groups tackling the different aspects involved in the reuse of wastewater coming from MWTPs.
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 lists the 9 compounds used. The heterogeneous photocatalytic degradation tests were carried out using a slurry suspension (5 mg L−1) of Degussa (Frankfurt, Germany) P-25 titanium dioxide (surface area 51–55 m2 g−1). Photo-Fenton experiments were performed using iron sulphate (FeSO4·7H2O),
Results and discussion
As the scope of this study was to propose a solar AOP treatment permitting reuse of MWTP effluents, the first step was to compare two different approaches, photo-Fenton and TiO2. In both cases tests were done under mild conditions (low iron and TiO2 concentration). Taking into consideration, as commented in the introduction, that typical concentrations are in the 0.1–20.0 μg L−1 range, it was decided to work at 100 μg L−1. It was also decided not to work (at least during the first stages of the
Conclusions
The experiments showed that emerging contaminants at low concentrations can be successfully degraded to negligible concentrations (a few μg L−1) with photo-Fenton at low iron concentration (5 mg L−1) and low initial H2O2. pH below 3 is not, in fact, a limiting factor for photo-Fenton to work properly, nor is the precipitation of iron. The main limitation to be overcome is the presence of CO32− and HCO3−, which are very efficient OH radical scavengers. The problems encountered with the scavenging
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 Junta de Andalucía (Project No. P06-TEP-02329). Nick Klamerth and Noelia Miranda like to thank the University of Almería and CIEMAT, for their Ph.D. research grants.
References (26)
- et al.
Chemosphere
(2008) - et al.
TrAC—Trends Anal. Chem.
(2007) - et al.
Trends Anal. Chem.
(2003) - et al.
Wat. Res.
(2009) - et al.
Water Res.
(2007) - et al.
Chemosphere
(2007) - et al.
Adv. Environ. Res.
(2004) - et al.
J. Environ. Manage.
(2007) - et al.
Catal. Today
(2007) - et al.
Water Res.
(2004)
Talanta
Water Res.
J. Catal.
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