Soil Component Interactions with 2,4-Dichlorophenoxyacetic Acid under Supercritical Fluid Conditions

Interest in using supercritical CO2 as an alternative to nonpolar liquid solvents to extract toxic organics from soils is growing. Unfortunately, supercritical CO2 alone is a poor solvent for many polar compounds, including acid herbicides, in soils. In this study, supercritical CO2 was modified with benzoic acid/methanol to extract 2,4-dichlorophenoxyacetic acid (2,4-D) from selected model soil components, analogs of soil components that potentially limit its extraction from soils. The components included four minerals, silica gel, sodium humate, and humic acid. These model materials were chosen to test three potential factors inhibiting 2,4-D extraction: (1) adsorption to mineral surfaces, (2) diffusion-limited release from porous materials, and (3) pH-dependent partitioning between the solid and supercritical fluid phases. High recoveries were obtained from gibbsite (100 ± 3%), goethite (91 ± 3%), and illite (88 ± 6%). Porous materials such as the silica gels 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. Overall, soil pH was the main chemical factor affecting 2,4-D recovery. Due to its porosity, pH buffering capacity, and ubiquitous occurrence, we contend organic matter will generally be the main component limiting extraction of 2,4-D from soils. Furthermore, it appears methanol enhances recovery, in part, because the protonated form of 2,4-D is favored 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 an ion pair is formed.