Effects of experimental variables on 2,4,5-T adsorption-desorption in soil

The effects of different experimental conditions on characteristics of weak acid pesticide adsorption-desorption in soil was examined using 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) as the test compound in a Palouse silt loam soil (Pachic Ultic Haploxerolls). The soil was equilibrated with 2,4,5-T solutions ranging in concentration from 0.1 to 196 μmol/L, using different ionic strengths of aqueous solution, soil/solution ratios, temperatures, and soil pretreatments. The data for all adsorption experiments fit the Freundlich equation and, depending on the experimental conditions, the adsorption constant K(f) ranged from 3.6 to 18.8, while 1/n varied from 0.80 to 0.92. Increasing the salt concentration of the aqueous solution from 0 to 0.1N CaCl₂ increased K(f) by a factor of 3.9. Changing the soil/solution ratio from 1:1 to 1:3.3 increased K(f) by a factor of 1.5. Decreasing the equilibration temperature from 35 to 5° C increased K(f) by a factor of 1.3. Preequilibration of the soil with 0.01N CaSO₄ without 2,4,5-T increased K(f) by a factor of 2.6. All changes in K(f) were statistically significant. Variations in K(f) values are sufficiently great that one could classify the mobility of 2,4,5-T in the Palouse soil as low or high. Desorption under all experimental conditions examined resulted in apparent hysteresis. However, the observed hysteresis may have partially resulted from the use of the batch equilibration method to conduct the desorption studies. Each desorption equilibration step apparently altered the physical and/or chemical properties of the soil-solution system in which the desorption occurred, compared with those in which the original pesticide-soil-solution adsorption equilibrium was evaluated.