Adsorption of the herbicide acifluorfen on soil humic acids

The amount of acifluorfen adsorbed on eight soil humic acids has been determined in a pH range from 1.6 to 6 at the initial concentration of 40 mg L(-1) using a batch equilibrium method. Most of the adsorption took place between pH 2.5 and 3.5. More than 70% acifluorfen was adsorbed at pH <2.5 and less than 10% at pH >4.5, indicating that adsorption was promoted by the protonation of both acifluorfen and humic acids. Adsorption isotherms were determined on six out of the eight humic acids at pH 3.4 in a concentration range from 4 to 100 mg L(-1) and described by the Freundlich equation. 1/n values were close to 1 indicating a linear ripartition between the liquid and solid phases. Four humic acids had about the same K-f (84.1-94.1), while the other two exhibited respectively a higher (126.8) and a lower (51.3) K-f. These results indicate that, beside pH, the nature of humic acid influences the adsorption of acifluorfen, but no correlation was observed between Kf values and the concentration of acidic functional groups of humic acids.