SOIL COMPONENT INTERACTIONS WITH 2,4-DICHLOROPHENOXYACETIC ACID UNDERSUPERCRITICAL-FLUID CONDITIONS

Interest in using supercritical CO2 as an alternative to nonpolar liqu id solvents to extract toxic organics from soils is growing. Unfortuna tely, supercritical CO2 alone is a poor solvent for many polar compoun ds, including acid herbicides, in soils. In this study, supercritical CO2 was modified with benzoic acid/methanol to extract 2,4-dichlorophe noxyacetic acid (2,4-D) from selected model soil components, analogs o f soil components that potentially limit its extraction from soils. Th e components included four minerals, silica gel, sodium humate, and hu mic acid. These model materials were chosen to test three potential fa ctors inhibiting 2,4-D extraction: (1) adsorption to mineral surfaces, (2) diffusion-limited release from porous materials, and (3) pH-depen dent partitioning between the solid and supercritical fluid phases. Hi gh recoveries were obtained from gibbsite (100 +/- 3%), goethite (91 /- 3%), and illite (88 +/- 6%). Porous materials such as the silica ge ls and humic acid yielded lower recoveries, 70 +/- 4% to 87 +/- 7% and 80 +/- 3%, respectively. We extracted only 11 +/- 2% of the spiked 2, 4-D from sodium humate. An inverse relationship existed between the pH of the solid-benzoic acid/methanol suspension and 2,4-D recovery. Ove rall, soil pH was the main chemical factor affecting 2,4-D recovery. D ue to its porosity, pH buffering capacity, and ubiquitous occurrence, we contend organic matter will generally be the main component limitin g extraction of 2,4-D from soils. Furthermore, it appears methanol enh ances recovery, in part, because the protonated form of 2,4-D is favor ed due to the higher pK(a) of 2,4-D in this solvent compared to water, since the ionized form will not dissolve in a nonpolar fluid unless a n ion pair is formed.