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

Supercritical fluid extraction (SFE) with carbon dioxide (SF‐CO 2 ) 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 d s) of atrazine in soils having water contents below field capacity. Low‐density SF‐CO 2 removed atrazine from soil solutions without first requiring separation of the solution from the soil. The K d s obtained by the SF‐CO 2 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 d s 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‐CO 2 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‐CO 2 method from desiccated soil though large amounts of water (16%) caused a dramatic increase in the K d values determined with SF‐CO 2 . The soil solution concentration at 4% soil water content related linearly to the inverse of the temperature ( T , K) and the isosteric heat (Δ H i ) was determined to be −55.2 ± 1.7 kJ mol ‐1 . The SF‐CO 2 method is promising as a technique to characterize herbicide sorption‐desorption from field‐moist soils.