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 adsor
bed at pH <2.5 and less than 10% at pH >4.5, indicating that adsorptio
n was promoted by the protonation of both acifluorfen and humic acids.
Adsorption isotherms were determined on six out of the eight humic ac
ids at pH 3.4 in a concentration range from 4 to 100 mg L(-1) and desc
ribed by the Freundlich equation. 1/n values were close to 1 indicatin
g 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 exhibit
ed 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 ads
orption of acifluorfen, but no correlation was observed between Kf val
ues and the concentration of acidic functional groups of humic acids.