MINERALIZATION, SORPTION AND DESORPTION OF CHLORPYRIFOS IN AQUATICSEDIMENTS AND SOILS
Gebremariam, Seyoum Yami
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Increased utilization of agrochemicals poses a substantial public health and ecological concern should they be transported to aquatic ecosystems. This dissertation presents a comprehensive literature review and results of a series of laboratory experiments investigating the mineralization, sorption and desorption of chlorpyrifos, a widely used organophosphorus insecticide. An extensive review of literature found that published sorption coefficients for chlorpyrifos in soils and sediments spanned up to two orders of magnitude. Normalizing the partition coefficient to organic content failed to substantially reduce variability, indicating the difficulty in predicting the sorptive behavior of chlorpyrifos without resorting to experimental methods specific to the soil or sediment of interest. Mineralization was studied using laboratory microcosms operated under drain-fill and permanently flooded conditions. Drain-fill cycling resulted in significantly lower mineralization rates relative to the permanently flooded condition due to enhanced partitioning of the pesticide to the sediment phase. Mineralization half-lives were 5-6 years and confirmed that chlorpyrifos and its metabolites can persist in aquatic environments for decades before they completely dissipate, particularly if sorbed to sediment. Sorption and desorption was studied using eight different aquatic soils and sediments using the batch equilibrium method. Sorption significantly correlated with organic content. The Kd values varied from 35.2 L/kg in low organic reservoir sediment to 123.3 L/kg in moderately organic cranberry soils. The Koc values varied from 3,606 to 5,983 L/kg. Desorption of chlorpyrifos exhibited hysteresis that increased with organic content and occurred in a dual mode with a labile and non-labile fraction. A mathematical model was used to determine the fraction of the labile component and the long-term fate of the non-labile component. The model quantified the long-term desorption of chlorpyrifos under repeated desorption events and predicted that contaminated aquatic soils and sediments can serve as a secondary and long-term source for chlorpyrifos pollution. Results also suggested that unfavorable energy changes during desorption, along with limitation on diffusivity of water molecules through the hydrophobic region of the organic fraction, may explain the observed hysteresis and biphasic nature of chlorpyrifos desorption.