Ultrasonic and photochemical degradation of chlorpropham and 3-chloroaniline in aqueous solution
Introduction
Several studies have already demonstrated that ultrasound is an efficient method for the degradation of organic pollutants in aqueous solution (Kotronarou et al., 1992; Serpone et al., 1994; Petrier et al., 1992b, Petrier et al., 1994). The applied pressure field induces the formation of microbubbles which oscillate and collapse. The implosion of the cavitation bubbles generates high energy reactions due to the local increase of temperature and pressure, that is the hot spot theory (Suslick and Hammerton, 1986, Suslick, 1988). The high energy phenomena can also be attributed to electrical discharges (Margulis, 1990) and/or corona effect (Lepoint and Mullie, 1994). In these conditions the homolytic cleavage of water yields OH and H which recombine or diffuse in the bulk.
Hydroxyl radicals lead to the oxidation of most of the organic compounds present in the solution. Besides, if the vapour pressure of substrates is sufficient for their diffusion in the short lived bubbles, a thermal degradation, assisted or not by electric discharges and/or corona effect also occurs.
If the ultimate purpose of sonochemistry is water treatment, photodegradation is one of the main natural processes of elimination of xenobiotic pollutants. It can also be used for the decontamination of polluted waters, in association with other processes: UV/H2O2, UV/O3 and photo-Fenton. The photolysis consists in exciting the substrate by absorption of a photon. Then several processes may be involved according to the structure of the compound. With chloroaromatic derivatives in dilute aqueous solution, photooxidation and heterolytic scission of the C–Cl bond with formation of the corresponding hydroxylated product and hydrochloric acid, are the most frequent reactions (Boule et al., 1982, Boule et al., 1985; Lipczynska-Kochany and Bolton, 1991).
The aim of the present work is to compare the ultrasonic degradation with the photodegradation of chlorpropham (also called CIPC) and 3-chloroaniline.
Chlorpropham (isopropyl-3-chlorocarbanilate) is a selective systemic herbicide and growth regulator. It is also used as a sprouting inhibitor for ware potatoes and sucker control agent in tobacco (Metcalf, 1971; Tsumura-Hasegawa et al., 1992; Tomlin, 1994). As its solubility in water is 80 mg l−1 and it is resistant to hydrolysis and oxidation, the bacterial degradation of chlorpropham is probably the dominant elimination pathway in the environment. It leads to the formation of the toxic 3-chloroaniline (Wolfe et al., 1978) listed on the European Community priority pollutant Circular No 90-55 (1990). So it is important to study and compare different processes to eliminate these compounds from water and analyse the intermediate products involved in their degradation.
Section snippets
Reagents
Chlorpropham (99%) was provided by Chem Service and used as received, 3-chloroaniline (>99%) by Aldrich, 3-aminophenol (>98%) and resorcinol (>99%) by Merck. Water used for solutions and HPLC was purified with a Milli-Q apparatus. Methanol was of HPLC grade.
Analyses
UV spectra of solutions were recorded on a CARY 13, Varian Spectrophotometer.
Separations and titrations of products were carried out on HPLC chromatographs (Waters 600E model and Beckman 420 model) equipped with C18 columns, 5 μm 250 mm×4.6 mm.
Ultrasonic transformation of chlorpropham
The disappearance of chlorpropham at 20 and 482 kHz is compared in Fig. 1.
The treatment at high frequency (initial rate 13.0×10−8 M s−1) is much more efficient than at 20 kHz (initial rate 4.2×10−8 M s−1). The complete degradation was obtained after 45 min at 482 kHz whereas about 1/3 of chlorpropham remained after 60 min at 20 kHz. The kinetics of formation of ionic species, 3-chloroaniline and formic acid at 482 kHz are reported in Fig. 2(a). Formation of 3-chlorohydroquinone has been observed on the
Ultrasonic transformation
Two mechanisms are involved in the ultrasonic transformation of chlorpropham and 3-chloroaniline. With chlorpropham, the initial formation of CO2 is too high to result only from the cleavage of carbamate function. Moreover, the transformation is partly inhibited by isopropanol. With 3-chloroaniline, the efficiency of the inhibition is higher than with chlorpropham and CO2 formed appears as a secondary sonoproduct.
The first mechanism involves hydroxyl radicals formed by sonolysis of water. It is
Conclusions
Ultrasound and light can transform chlorpropham and 3-chloroaniline, but the intermediates are different.
The ultrasonic degradation is more efficient at 482 kHz than at 20 kHz for both compounds. With chlorpropham, intermediates are hydroxylated products and 3-chloroaniline. Two pathways are involved, the oxidation by ̇OH and pyrolysis near the cavitation bubbles. 3-chloroaniline, which is a toxic intermediate, does not accumulate and is mineralised.
Ultrasonic treatment at 482 kHz appears to be an
Acknowledgements
The authors are grateful to G. Keravis (Université d'Orléans) and V. Jacob (Université de Grenoble, GRECA) for their help in mass spectrometry. They also thank the Service Central d'Analyse du C.N.R.S. for GC-MS analyses.
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