Ozonization of Acetochlor: Kinetics, By-Products and Toxicity of Treated Aqueous Solutions

M. Kene and J. Kurnik

Abstract
Ozonization of aqueous solutions of acetanilide type pesticide (acetochlor) was studied in a batch reactor operated at temperatures between 20 and 47 °C and ozone partial pressures from 236 to 488 Pa. Acetochlor mass concentrations were in a range between 1 and 200 mg L–1. When the pH value of solutions was 5 or below, acetochlor decomposition was a result of its direct reaction with ozone, and the pesticide-decay data were well described by power-law kinetics: first-order with respect to acetochlor and 0.37 with respect to ozone concentration. The activation energy for the pesticide oxidation was found to be 67 kJ mol–1. While acetochlor could be completely removed from the solutions, the total organic carbon (TOC) reduction was appreciably lower (35 %) due to the formation of ozone-resistant intermediates. The kinetics of TOC concentration reduction was found to obey a first-order process with two distinct steps, with an activation energy of 33 kJ mol–1 for the first step and 44 kJ mol–1 for the second step. Ozonization was also more effective under alkaline condition with addition of a co-oxidant such as Fenton reagent or hydrogen peroxide. The ability of ozone to oxidize the molecular structure of dissolved acetochlor resulted in an increase of biodegradability, demonstrated by an increase in the BOD5/COD ratio, as well as in a reduced level of toxicity determined by the Daphnia magna test.

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Keywords
Ozonization, Pesticides, acetochlor, kinetics, toxicity, biodegradability