Environ. Sci. Technol. 2000, 34, 1563-1571
Photocatalytic Pilot Scale Degradation Study of Pyrimethanil and of Its Main Degradation Products in Waters by Means of Solid-Phase Extraction Followed by Gas and Liquid Chromatography with Mass Spectrometry Detection ANA AGU ¨ ERA, EVA ALMANSA, ANA TEJEDOR, AND A M A D E O R . F E R N AÄ N D E Z - A L B A * Pesticide Residue Research Group, University of Almerı´a, 04071 Almerı´a, Spain SIXTO MALATO AND MANUEL I. MALDONADO Plataforma Solar de Almerı´a-CIEMAT, Crta. Sene´s, Km 4, 04200 Tabernas, Almerı´a, Spain
Aqueous solutions of Pyrimethanil, as technical grade product (TP), with 98.2% purity, and commercial formulation (CF), containing 40% (w/v) of Pyrimethanil, were submitted to photocatalytic degradation under sunlight in the presence of TiO2 as catalyst in a preindustrial pilot plant. Complete Pyrimethanil degradation was achieved after ca. 230 min of irradiation, in both TP and CF, but total mineralization was not observed, as was demonstrated by the TOC values of 3-4 mg/L, measured at the end of the experiments (907 min). A qualitative and quantitative study of the degradation products (DPs) generated during the process was performed by GC-MS, using EI and CI as ionization modes, and by LC-API-MS, using Atmospheric Pressure Chemical Ionization (APCI) and Electrospray (ES) interfacing techniques. Up to 22 compounds could be detected as degradation intermediates. To evaluate the extraction efficiency of these DPs from the aqueous solution, specially for the more polar intermediates, a recovery study was performed with Pyrimethanil and seven of the commercially available DPs. Liquid-liquid extraction (LLE) and solidphase extraction (SPE), with different sorbents, were compared. A SPE method using Lichrolut-EN cartridges was selected as the most adequate, but recoveries e60% were obtained for four of the DPs studied (aniline, formamide, 1,3-benzenediol, and 4,6-dimethyl-2-pyrimidinamine). Structure identification of DPs allowed us to propose two main routes in the degradation process. One route involves the attack of hydroxyl radicals to the pyrimidine and benzene rings with further rings opening and the other one corresponds to a photoinduced hydrolysis of the molecule by the amine group bonds.
Introduction The generation of aqueous wastes related with pesticide industrial and agricultural activities can be considered 10.1021/es990112u CCC: $19.00 Published on Web 03/15/2000
2000 American Chemical Society
unavoidable. Besides the traditional nondestructive treatment methods to remove these substances, different destructive technologies for pesticide water treatment have been of growing concern over the past decade. Chemical and photochemical degradation treatments of pesticides present in wastewaters are mainly based on hydroxyl radical (OH•) generating systems which initiate a sequence of degradative reactions resulting in the total or partial destruction of pollutants in waters (1, 2). Many of these systems (e.g. O3, UV/O3, photocatalysis, Fenton’s reagent) have been widely tested for a large variety of pesticides. Titanium dioxide (TiO2) is the most widely accepted photocatalyst for pesticide destruction in water due to (i) a high production capacity of hydroxyl radicals; (ii) the spectral characteristics of TiO2 which allow its excitation in a broad UV spectral range including sunlight spectrum; and (iii) the low price of this compound. Nevertheless, only few experiments have been performed and evaluated on a pilot scale (3, 4). Ideal treatments would achieve the complete mineralization of the pesticide wastes in a rapid and nonselective way, but complete degradation to inorganic products is not always feasible and the presence of byproducts or degradation products (DPs) appears to be unavoidable in many cases. Afterward many of the possible DPs produced may have toxicological characteristics similar to or higher than the parent compounds (5). As a consequence, an extensive analytical evaluation of these oxidative degradation procedures is necessary to identify as many DPs as possible in order to (i) evaluate the treatment efficacy; (ii) achieve the fine-tuning of the experimental parameters which avoid the production of toxic derivatives; and (iii) perform complete kinetic studies. Since there is a lack of degradation product standards, it is difficult to identify pesticide degradation pathways through “conventional” chromatographic detectors (2). The use of mass spectrometry (MS) coupled with gas and liquid chromatographic systems represents the main alternative to carry out the identification of the pesticide DPs generated under these degradative procedures (6). This fact is, even when a definitive assignment of chemical structures is not always feasible, a consequence of the short analysis time required with respect to other analytical techniques or procedures and the higher sensibility achieved by these techniques with respect to others like IR, NMR, etc. Therefore, the interest in these degradation studies for the combination of both GC-MS and LC-MS based techniques is growing (6). Taking into account the polar derivatives we can expect (e.g. hydroxy derivatives, short chain acid derivatives) and the typical drawbacks of the derivatization procedures to convert these compounds in GC amenable ones, the use of hyphenated LC-MS is clearly favored. Also, new electrospray (ES) or atmospheric-pressure chemical ionization (APCI) LC-MS interfacing systems have extended the applicability of LCMS in these studies mainly because of the high sensitivity and structural information that can be obtained (7). Nevertheless, the higher sensitivity and discriminating power of GC with respect to LC as well as the development of new low bleeding capillary polar columns make GC-MS techniques necessary when the number of DPs produced is high and coelution peaks are unavoidable in typical LC columns or when the sensitivity requirements are high * Corresponding author phone: +34 50 21 50 34; fax: +34 50 21 54 83; e-mail:
[email protected]. VOL. 34, NO. 8, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scheme of the solar photoreactor used for the TiO2 photocatalytic degradation of Pyrimethanil in water solution. enough. In such a situation both GC-MS and LC-MS can be considered necessary and complementary. Although liquid-liquid extraction (LLE) using an appropriate solvent (8-10) is usually the sample handling procedure of choice, other strategies based on the use of solid-phase extraction (SPE) are gaining acceptance, mainly because new SPE sorbents are able to trap DPs of a large range of polarities with low matrix interferences (11). Afterward the use of sequential solid-phase extraction (SSPE) using different sorbent cartridges in tandem (e.g. octadecylsilica C18 and polymeric Lichrolut EN) to obtain fractionated extracts can simplify greatly the subsequent GC or LC analysis (12). In the present work, a study of the photocatalytic degradation of Pyrimethanil [N-(4,6-dimethylpyrimidin-2yl)aniline)] has been carried out.
Pyrimethanil can be used to evaluate the degradation efficacy in a pilot scale photoreactor, as it is a nonsystemic insecticide with a water solubility of 0.12 g/L (pH 6.1, 25 °C) containing an interesting chemical structure provided with benzene and pyrimidine rings. In this work we have applied various sample handling procedures based on LLE and SPE followed by an extensive application of GC-MS and LC-MS techniques to provide a detailed study and evolution of Pyrimethanil and its main DPs produced under TiO2 photocatalysis in a pilot scale solar photoreactor consisting of compound parabolic collectors (CPCs) (Figure 1).
Experimental Section Chemicals. Agrevo S.A. (Alca´cer, Spain) supplied Pyrimethanil technical grade (98.2% purity). Pyrimethanil commercial formulation (Scala SC, Agrevo S.A.) was purchased locally. This solution contained 40% (w/v) of Pyrimethanil dispersed in an emulsion of unknown matrix. Standards of the following compounds were used: 1,3-dihydroxybenzene (98%) and 1,4-dihydroxybenzene (99.5%) from Riedel-de Hae¨n (Hannover, Germany) and 2-amine-4,6-dimethyl-pyrimidine (96%), aniline (99.5%), formanilide (98%), acetanilide (99%), and formamide (99%) from Fluka (Buchs, Switzerland). The solvents used in this work were as follows: pesticidegrade dichloromethane, ethyl acetate, and acetone from Scharlau (Barcelona, Spain) and HPLC-grade water, acetonitrile, and methanol from Merck (Darmstadt, Germany). All photocatalytic degradation assays were carried out using titanium dioxide Degussa P-25 (Frankfurt, Germany), with a surface area of 51-55 m2 g-1, as photocatalyst. The water used in the experiments was obtained from the PSA Desalination Plant (evaporation by a multieffect system using solar energy, conductivity
