SUPERCRITICAL-FLUID EXTRACTION OF 2,4-D FROM SOILS - PH AND ORGANIC-MATTER EFFECTS

Supercritical fluid extraction (SFE) recoveries of 2,4-D (2,4-dichlo r ophenoxyacetic acid) from spiked Washington soils depended on both pH and organic matter content. The soils used in this study spanned a wid e range of organic C concentration (2-480 g kg(-1)). One of the soils (the Benge series A horizon) was adjusted to pHs from 1.8 to 8.3, and extracted with methanol-modified supercritical CO2. Recoveries from th is soil increased from 41 to 76% with decreasing pH. For soils at natu ral pHs, when a weak acid modifier (benzoic acid) was used in addition to methanol, the 2,4-D recovery ranged from 21 to 86%, increasing wit h decreasing organic C concentration. Treating the samples to the lowe st pHs practical (1.2-1.6) with HCl and extracting with methanol-modif ied supercritical CO2 was more successful than utilizing the weak acid -methanol modifier, except for the soil with very low organic C. Recov eries after HCl treatment ranged from 80 to 94%. Two soils from 2,4-D- treated fields yielded more 2,4-D by modified SFE at pH 1.2 than by a modified version of the method recommended by the U.S. Environmental P rotection Agency. The results suggest that organic acids like 2,4-D ca n be successfully extracted at low pH, even from soils high in organic C. Low recoveries from soils high in organic C appear to result from proton consumption by natural soil organic matter during extraction.