Transport and sorption of 2,4-dichlorophenoxyacetic acid in allophanic soils
2007
Müller, Karin | Duwig, Céline
Field applications of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) are commonly formulated as esters, amine salts, or alkali, but studies on the environmental impact focus on the 2,4-D acid form. We hypothesized that the formulation would affect its transport. To better understand 2,4-D transport through allophanic soils, leaching experiments with 2,4-D acetic acid, 2,4-D ethylhexyl ester, and tritium were conducted in 100 mm of repacked soil columns. Three of the four silt loam soils were allophanic. The 2,4-D sorption isotherms and kinetics of sorption were determined from batch experiments. The symmetrical tritium breakthrough curves (BTCs) showed that water transport was in physical equilibrium, and these were used to estimate the dispersivity of the soils. Tritium was not inert in the allophanic soils. The BTCs of both 2,4-D formulations were shifted to the right of tritium and showed extended tailing characteristic typical for sorption nonequilibrium. The 2,4-D BTCs were successfully fit with a two-site chemical nonequilibrium model. Batch and leaching experiments showed the sorption of 2,4-D ethylhexyl ester to be higher and characterized by stronger nonequilibrium than 2,4-D acetic acid sorption in all soils. The sorption coefficient values derived from the BTCs were higher than those from 24-h batch experiments. For the two allophanic topsoils, the predicted ester BTCs were significantly worse when using the independently derived Kd compared with the BTCs using the fitted Kd. 2,4-D sorption was correlated to pH and organic matter content. Transport of 2,4-D was slower than expected in the allophanic soils and was attributed to retention of dissociated 2,4-D by positively charged soil surfaces.
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