Supercritical carbon dioxide for determining atrazine sorption by field-moist soils

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(d)s) 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(d)s 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(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-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 Kd 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 (Delta 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.