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Case Study in Environmental Chemistry....
Case Study 3: Photolysis of Terbufos
Authors: Sarunya Hengpraprom and Cindy Lee, Environmental Engineering and Science, Clemson University.
Abstract : This case study considers the rate at which exposure to sunlight can transform a pesticide known as terbufos. Transformation of a contaminant by photolysis will change its behavior in a given situation. In some cases, a product of photolysis is just as toxic or more toxic than the parent compound. In other cases, the products of photolysis are rendered nontoxic and the transformation can be considered beneficial. The experiment described in this case study produced some of the first basic kinetics data available to evaluate the importance of photolysis as a process to remove terbufos from the environment. For more detailed information about this research, see Lee, C. M.; Anderson, B.; and Elzerman, A. W. 1999. Photochemical oxidation of terbufos. Environmental Toxicology and Chemistry. 18(7):1349-1353.
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Experimental Approach and Method
The photolysis studies of terbufos were conducted in both unbuffered and buffer deionized-distilled water (DDI) matrices at three pH values (5, 7, and 9). The buffer solutions employed in this experiment consisted of a 0.05 M potassium acid phthalate (KHC8H4O4) and 0.023 M sodium hydroxide (NaOH) solution;- a 0.05 M potassium dihydrogen phosphate (KH2PO4) and 0.03 M NaOH solution;- and a 0.013 M borax (Na2B4O7) and 0.005 M hydrochloric acid (HCl) solution for adjusting the pH to 5, 7, and 9, respectively. Sample solutions of terbufos were freshly prepared at concentration of 1.91 x 10-3 mol/L by diluting the appropriate amount of stock solution (terbufos in acetone) with DDI water or buffer solution. All sample solutions contained less than 2 % (v/v) acetone. Sample solutions were transferred into 11 x 100-mm quartz vials, and then sealed with polyethylene caps. The vials were irradiated by natural sunlight. Irradiation of samples was conducted at approximately 35o north latitude and 83o west longitude near Clemson, South Carolina, USA. Sample vials were locked in painted non-reflective black racks that held them at an angle of 30 degree from the horizontal oriented in a north-south manner, with the tops of each vials pointing north. Temperature and pH of the sample solutions were monitored throughout the studies. Duplicate samples were removed and extracted by a liquid-liquid extraction method. Extract samples were refrigerated for later analysis by gas chromatography.Dark control studies were run to determine the transformation rate of aqueous terbufos in the absence of sunlight energy. Identical samples were prepared and placed in quartz vials wrapped in aluminum foil. The vials were then stored in a dark cabinet at room temperature and sampled at selected time intervals. At subsequent time intervals, duplicates of the terbufos solution were removed and extracted by a liquid-liquid extraction method. Extract samples (in hexane) were analyzed by gas chromatography (GC). The buffer solutions, quartz vials, and necessary glassware employed in this experiment were autoclaved prior to each study to minimize biodegradation. Temperature and pH were measured throughout the studies.
The absorption spectrum of terbufos was quantified by a ultraviolet-visible spectrophotometer. Double beam mode was used as a standard for all spectra measurement. Both a deuterium and a tungsten lamp were employed as light sources. The deuterium source provided light energy in the 200-370 nm wavelength ranges, whereas the tungsten source provided for 350-800 nm wavelength ranges. Quartz sample cells of either 1 cm or 5 cm light path length were utilized to accumulate ultraviolet (< 400 nm wavelength) light transmission. The absorbance accuracy of the instrument was verified according to the U.S. Environmental Protection Agency procedures (15 USEPA, 1985). Aqueous concentrations of terbufos were 0.24, 1.18, and 0.25 ppm in buffer solutions prepared at pH 5, 7, and 9, respectively.