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Supercritical Carbon Dioxide for Determining Atrazine Sorption by Field-Moist Soils

Dep. of Plant, Soil, and Entomological Sciences, Univ. of Idaho, Moscow, ID 83844-2339

William C. Koskinen^*

USDA-ARS, Soil and Water Management, 1991 Upper Buford Circle, Univ. of Minnesota, St. Paul, MN 55108

*Corresponding author (koskinen{at}soils.umn.edu).

ABSTRACT

Supercritical fluid extraction (SFE) with carbon dioxide (SF-CO₂) has been used effectively as a method for the extraction of pesticides from soils. This study was intended to test its potential as a means of removing atrazine from soil water for quantification, to allow calculation of sorption coefficients (K_ds) of atrazine in soils having water contents below field capacity. Low-density SF-CO₂ removed atrazine from soil solutions without first requiring separation of the solution from the soil. The K_ds obtained by the SF-CO₂ method for the topsoil and the lower root zone samples were 1.21 ± 0.04 and 1.14 ± 0.03, respectively, while that of the vadose zone soil was 0.16 ± 0.00. Desorption was rapid; equilibrium was reattained within 7 min. Desorption K_ds 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. The SF-CO₂ method 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 SF-CO₂ method from desiccated soil though large amounts of water (16%) caused a dramatic increase in the K_d values determined with SF-CO₂. The soil solution concentration at 4% soil water content related linearly to the inverse of the temperature (T, K) and the isosteric heat (H_i) was determined to be –55.2 ± 1.7 kJ mol^-1. The SF-CO₂ method is promising as a technique to characterize herbicide sorption-desorption from field-moist soils.

Received for publication February 6, 1995.