COMPARATIVE PHOTODEGRADATION RATES OF ALACHLOR AND BENTAZONE IN NATURAL-WATER AND DETERMINATION OF BREAKDOWN PRODUCTS

The photochemical degradation of the herbicides alachlor (2-chloro-2'6'-diethyl-N-methoxymethylacetanilide) and bentazone (3-isopropyl-(1H)-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide) in distilled water and in river water, under xenon are lamp irradiation, was investigated. Analytical determinations were carried out by using a xenon are photoreactor and on-line solid-phase extraction coupled to liquid-chromatography diode-array and liquid-chromatography mass-spectrometry detection systems. Photolysis experiments were performed at low concentration (20 mu g/L), and the advantages of this methodology over conventional techniques are discussed. The photodegradation of alachlor and bentazone is a process depending on the water type, humic substances, and pH. When using a solution of 4 mg/L of humic matter, the estimated alachlor and bentazone half-lives were 84 and 150 min, respectively, using a total irradiance of 550 W/m(2) in the range of 300 to 800 nm. The degradation of alachlor and bentazone followed pseudo second- and first-order kinetics, respectively. The major photodegradation products of both herbicides were identified either by on-line solid-phase extraction (SPE)-liquid chromatography-thermospray mass spectrometry (LC/TSP-MS) or on-line-SPE-LC/TSP-tandem mass spectrometry (LC/TSP-MS/MS). In addition to that, a total of six transformation products of alachlor were synthesized in our laboratory and their MS spectra were compared with those of the breakdown products obtained. After photodegradation, a total of 14 photoproducts resulted from alachlor dechlorination with subsequent hydroxylation and cyclization processes. The two major photoproducts were identified as hydroxyalachlor and 8-ethyl-1-methoxymethyl-4-methyl-2-oxo-1,2,3,4-tetrahydroquinone. No significant breakdown products of bentazone could be identified.