Adsorption of chloroacetanilide herbicides on soil and its
components III. Influence of clay acidity, humic acid coating and herbicide
structure on acetanilide herbicide adsorption on homoionic clays
Affiliations: Institute of Environmental Science, Zhejiang
University, Hangzhou 310027, China | Institute of Industry Environmental Protection,
Zhejiang Province, Hangzhou 310005, China
Note: [] Corresponding author
Abstract: Adsorption of chloroacetanilide herbicides on homoionic
montmorillonite, soil humic acid, and their mixtures was studied by coupling
batch equilibration and FT-IR analysis. Adsorption isotherms of acetochlor,
alachlor, metolachlor and propachlor on Ca^2+_-, Mg^2+_-. Al^3+_ - and Fe^3+_ -
saturated clays were well described by the Freundlich equation. Regardless of
the type of exchange cations, K_f decreased in the order of metolachlor >
acetolachlor > alachlor > propachlor on the same clay. FT-IR spectra
showed that the carbonyl group of the herbicide molecule was involved in
binding, probably via H-bond with water molecules in the clay interlayer. The
type and position of substitutions around the carbonyl group may have affected
the electronegativity of oxygen, thus influencing the relative adsorption of
these herbicides. For the same herbicide, adsorption on clay increased in the
order of Mg^2+ < Ca^2+ < Al^3+ < Fe^3+ which coincided with the
iucreasing aciditv of homoionic clays. Acidity of cations may have affected the
protonation of water, and thus the strength of H-bond between the clay water
and herbicide. Complexation of clay and humic acid resulted in less adsorption
than that expected from independent adsorption by the individual constituents.
The effect varied with herbicides, but the greatest decrease in adsorption
occurred at a 60:40 clay-to-humic acid ratio for all the herbicides. Causes for
the decreased adsorption need to be characterized to better understand
adsorption mechanisms and predict adsorption from soil compositions.
