Solar photocatalytic degradation of some hazardous water-soluble pesticides at pilot-plant scale
Introduction
During the last few decades, the development of the agrochemical industry has dramatically increased due to widespread intensive agriculture. Consequently, human health is threatened by high concentrations of pesticides, heavy metals, hydrocarbons, chlorinated hydrocarbons, etc., present in continental and marine natural waters as a result of low-to-medium contaminant concentrations (<500 mg L−1) in industrial effluents. EU legislation is constantly being adapted to protect and improve the quality of Europe's fresh water resources [1].
Most pesticides require more effective treatment systems due to their high chemical stability and/or low biodegradability [2], [3], [4]. Among these persistent water contaminants are the six water-soluble pesticides studied in this work: cymoxanil, methomyl, oxamyl, dimethoate, pyrimethanil and telone (of special interest because of their extremely easy transport in the environment).
The strong potential of advanced oxidation processes (AOPs) for bio-recalcitrant wastewater treatment is universally recognized today. Many studies have focused on the development of these processes using different reaction systems [5], [6], [7]. AOPs are characterized by the production of hydroxyl radicals (OH), the second strongest known oxidant after fluorine (2.8 V versus standard hydrogen electrode). It is able to oxidize and mineralize almost every organic molecule, yielding CO2 and inorganic anions. Many oxidation processes, such as TiO2/UV, H2O2/UV, photo-Fenton and ozone (O3, O3/UV, O3/H2O2) are currently employed. The advantage of AOPs is enhanced by the fact that there are different OH radical production possibilities, so they can be adapted to specific treatment requirements. On the other hand, the relatively high operating costs of these processes compared to those of a biological treatment are the main disadvantage. The use of AOPs for wastewater treatment has been studied extensively, but UV radiation generation by lamps or ozone production is expensive. Therefore AOPs which can be driven by solar radiation (photo-Fenton and heterogeneous catalysis with UV/TiO2) are of special interest, making the development of suitable technologies for practical applications very attractive [8], [9].
Different semiconductor materials have been tested under comparable conditions for the degradation of the same compounds, among which TiO2 has generally been demonstrated to be the most suitable [10]. In the photo-Fenton process, the Fenton reagent produces OH radicals by the addition of H2O2 to Fe2+ salts. The organic pollutant degradation rate is strongly accelerated by irradiation with UV–vis light in photo-Fenton [11]. Under these conditions, the photolysis of Fe3+ complexes promotes Fe2+ regeneration and iron may be considered a true catalyst.
This work evaluates these two well-defined AOP systems in two different pilot-plant photoreactors illuminated by sunlight. The efficiency of both treatment methods in the degradation of six medium to highly-soluble pesticides (cymoxanil, methomyl, oxamyl, dimethoate, pyrimethanil and telone) was evaluated. These contaminants, belonging to well-known pesticides families (organophosphorous, organochlorines, etc.), were selected due to their wide application in agriculture and their high water solubility (of special interest because of their extremely easy transport in the environment).
Section snippets
Chemicals
Cymoxanil (98.2% technical grade C7H10N4O3, Aragonesas Agro S.A.), methomyl (99.4% technical grade C5H10N2O2S, Aragonesas Agro S.A.), oxamyl (Vydate 24% commercial grade C7H13N3O3S, Du Pont Iberica S.A.), dimethoate (98.2% technical grade C5H12NO3PS2, Aragonesas Agro S.A.), pyrimethanil (98.2% technical grade C12H13N3, Agrevo S.A.) and telone (90% technical grade C3H4Cl2, 1,3-dichloropropene, Aldrich Chemical) were used as received (Scheme 1). Analytical standards for chromatography analyses
Results and discussion
First, the original toxicity and biodegradability of the target pesticides (50 mg L−1) were analysed to justify using AOPs (TiO2 and photo-Fenton processes) for pesticide degradation rather than a conventional biological treatment. Table 2 shows the results of toxicity assays and Zahn–Wellens tests (DBO5 for telone) carried out for each pesticide. It may be observed that all the tested compounds are toxic at concentrations of less than 50 mg L−1 except telone (toxic at concentrations >85 mg L−1).
Conclusions
It has been demonstrated that wastewater contaminated by six medium to highly soluble pesticides (cymoxanil, methomyl, oxamyl, dimethoate, pyrimethanil and telone), can be treated successfully by photo-Fenton and TiO2 photocatalytic processes, within a reasonable length of time. Photo-Fenton treatment was more efficient than TiO2, not only for pesticide degradation, but also for TOC mineralization, except for dimethoate for which the mineralization rate was quite similar in both photocatalyitic
Acknowledgments
The authors wish to thank the Spanish Ministry of Education and Science for its financial assistance under the “Fotodetox” Project (PPQ 2003-07596-C03-01). They also wish to thank Mrs. Deborah Fuldauer for correcting the English. Isabel Oller would like to thank the Ministry of Education and Science for her PhD research grant.
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