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DISCUSSION For both reactors tested, the atrazine data appear to fit a first-order model, but the flat-plate reactor is a closer approximation of the safer, cheaper kind of low light intensity (1-10 x 10-9 einsteins/sec cm2) system that will be practical for scale-up. Using the rate equation determined by linear regression on the logarithmically transformed data, the half-life for atrazine in this system is approximately 4000 minutes. To lower an initial 10 ppm concentration of atrazine to below 10 ppb, degradation must proceed for 10 half-lives. However, utilizing an annular design of 1.8 cm diameter x 1 m long with a standard 40W fluorescent black light will increase the light intensity by a factor of six and the illuminated surface area by a factor of 45. With such a reactor, the t1/2 for atrazine would be approximately 15 minutes. With a starting concentration of 10 ppm, about 2.5 hr would be required to bring the atrazine concentration down to 1 0 ppb, and could continuously process liquid at a rate of 1 mL/min. An additional 50-60 minutes would be required to reduce the concentration to less than 1 ppb, for a flow rate of 0.75 mL/min. Electric power costs (at 60/kWh) work out to $1.60/gallon or $88 to treat the liquid in a 55 gallon drum to the 1 0 ppb level. Formic acid degradation did not appear to follow first-order kinetics, but the rate was about 49 times faster than the zero-time rate for atrazine. It is expected that the degradation rates of most compounds will fall within these bounds. Use of this technology in treating drinking water contaminated with pesticides might not be accepted by the public unless the levels could be decreased to 10% of Wisconsin's Preventative Action Limit (PAL). For atrazine, the PAL is 0.3 ppb and therefore 0.03 ppb is likely to be an acceptable level. For a well contaminated at the enforcement standard (3 ppb), about seven half-lives would be needed. Many studies have shown the diminished phytotoxicity of the dechlorinated, hydroxy-analogs of atrazine, and in the standard setting process in Wisconsin that only chlorinated metabolites are included in the state standards. Thus the assertion appears to have been accepted that hydrolysis reduces the human toxicity of atrazine 19
and its chlorine-containing metabolites. These calculations suggest that photocatalytic degradation is a relatively slow process, but faster degradation is expected with the titania pellets. Improvements in catalyst efficiency and optimization of reactor design promise to increase the rate significantly. Earlier studies in this laboratory indicate that improvements in quantum efficiency may be obtained with platinum doping. A pellet-filled reactor similar to that described above would be inexpensive to build and operate. Current work is now focusing on degradation of mixed pesticide rinsates collected from a commercial applicator in Wisconsin and of other pure pesticides. Experiments are being designed to determine rate equation parameters for compounds of interest and for mixed wastes. The use of hydrogen peroxide and other electron acceptors in these systems is also of interest. 20