Doctor of Philosophy (PhD)
Plant, Environmental Management and Soil Sciences
Adsorption-desorption and transport behavior of pesticides are important processes that influence the amount retained by the soil and that which is susceptible to runoff or movement within the soil profile. Kinetic batch experiments were used to study the adsorption-desorption behavior of metolachlor (a non-ionic herbicide) and deltamethrin (a pyrethroid insectcide) in soils, and atrazine (a triazine) in sugarcane mulch residue. Miscible displacement experiments under steady flow conditions were carried out to examine the mobility of these pesticides in soils. Metolachlor adsorption and desorption by Sharkey clay soil(Very-fine, smectitic, thermic Chromic Epiaquerts) and Commerce silty loam soil (Fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Epiaquepts) soil were strongly kinetic. The Kd values of metolachlor adsorption for Sharkey were significantly higher than for Commerce. After six desorption steps, the average metolachlor desorbed was 30.5% and 43.9% for Sharkey and Commerce, respectively, and was dependent on the initial input concentration. Adsorption and desorption of atrazine by sugarcane mulch exhibited extensive kinetic behavior. Adsorption isotherms appeared linear for all reaction times. A linear multireaction model which accounted for reversible equilibrium and kinetic retention along with a consecutive irreversible reaction described both adsorption and desorption of atrazine retention results (r2=0.99). Deltamethrin retention by soils was strong, and positively related with cation-exchange capacity (CEC) and negatively related with soil pH. Deltamethrin recovery was dependent upon the initial input concentration, and did not exceed 10.2% of the total amount adsorbed for all the four soils (Sharkey clay, Mahan sand loam, Vacherie silty loam and Mhoon silt clay loam) representing a wide range of organic matter and clay contents. Extremely low mobility and thus strong retention for deltamethrin in the soil columns were observed. A new hysteresis coefficient based upon the difference in the area between adsorption and desorption isotherms was proposed. This coefficient was capable of quantifying hysteresis for both types of desorption isotherms: traditional desorption isotherms and time-dependent desorption isotherms. Results suggested that when an isotherm is measured based upon laboratory adsorption-desorption experiments or field measurements, equal reaction times should be maintained.
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Zhu, Hongxia, "Retention and movement of reactive chemicals in soils" (2002). LSU Doctoral Dissertations. 2498.
H. M. Selim