Chlorothalonil in a Quartz Sand Soil: Speciation and Kinetics

The distribution of chlorothalonil among the dissolved, labile sorbed, and bound residue states was monitored during an 18 day period in an aqueous slurry of an analyzed quartz sand soil from Simcoe, ON, Canada. The Simcoe soil is 90.−95.% quartz sand. The online HPLC microextraction method was used for this purpose, because it is the only available technique that can resolve the total amount of a pesticide in a soil into its dissolved, labile sorbed, and bound residue components. The processes for which the molecular level kinetics were determined included labile surface sorption and desorption and bound residue formation. At a reaction time of 14 days, the solution concentration of 0.75 × 10-6 M was 43.3% of the total chlorothalonil, 26.2% was in the labile sorbed state, and 30.5% was a bound residue. There were no chemical reactions and no biodegradation during the 18 day period. The kinetics of mass transfer among the three states were determined and are consistent with intraparticle diffusion. Although the amounts are small, it is suspected that the 5.−10.% nonquartz materials in the Simcoe soil contribute most of the sorption and bound residue effects.