Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 7, 1995
Publication Date: N/A
Interpretive Summary: Knowledge of the uptake and retention (sorption) of herbicides by soils is essential for predicting the rate and extent of herbicide leaching through soils to ground water. Sorption coefficients (Kds) are a measure of how much herbicide is taken up by the soil relative to how much remains in the water in contact with the soil. In the past it was necessary to remove the water from the soil for herbicide analysis. The amount of herbicide remaining in the soil was calculated by subtracting the amount of herbicide in the water from the amount of herbicide applied. Kds could only be measured by wetting soils to as high as 100% water content (1 part water to 1 part soil) or higher to allow sufficient water for removal and analysis. Herbicide-soil Kds for drier soils, as are typical in the field, had not been determined. In this study we developed a technique using compressed, heated carbon dioxide gas for the removal of the herbicide atrazine from the soil water of soils having only 4 to 5% water content, without requiring removal of the water itself. We could then calculate herbicide Kds in soils at water contents observed in the field. Also, with our new technique the effect of soil temperature on herbicide sorption could easily be determined. The results indicate that, compared to traditional techniques, our new technique is a more convenient, precise, rapid, and ultimately more accurate means of characterizing herbicide sorption/desorption, to aid in predicting leaching of herbicides in soils of typical field moisture.
Technical Abstract: The use of supercritical fluid carbon dioxide (SF-CO2) was examined as a means of obtaining sorption coefficients (Kds) for topsoils with only 4 to 5% soil moisture. SF-CO2 removed atrazine from soil solutions without first requiring separation of the solution from the soil. Kds from the SF-CO2 method for the topsoil and the lower root zone samples were 1.21 and 1.14, respectively, while that of the vadose zone soil was 0.16. Desorption was rapid; equilibrium was reattained in 7 min. Desorption Kds for the topsoil and lower root zone soil were constant for successive desorption equilibrations, through removal of approximately 25% of the applied atrazine from the system. Batch sorption isotherms for all three soils fit the Freundlich equation and desorption was hysteretic. The lower root zone soil mean sorption Kf and 1/n values, 1.09 and 0.93, respectively, were the same as for the topsoil, 1.09 and 0.95, respectively. The vadose zone Kf and 1/n values were 0.37 and 1.02, respectively. SF-CO2 can be used to determine the effect of changes in water content and temperature on sorption. It was found that little atrazine can be extracted by the SFE method from dry soil, suggesting that water may provide a source of "easily extractable" atrazine, though large amounts of water (16%) caused a dramatic increase in the Kd values determined with SF-CO2. The soil solution concentrations at 4% soil moisture related linearly to the inverse of the temperature (degree K) and the isosteric heat (of sorption) was determined to be -13.2 kcal mole**-1, which is more negative than values previously reported in the literature for atrazine. The results indicated that using supercritical CO2 is promising as a technique to characterize herbicide sorption/desorption