Adsorption of mecoprop and dichlorprop on calcareous and organic matter amended soils:: Comparative adsorption of racemic and pure enantiomeric forms

The adsorption isotherms of mecoprop [R,S-2-(4-chloro-2-methylphenoxy) propanoic acid] and dichlorprop [R,S-2-(2,4-dichlorophenoxy)propanoic acid] on twelve calcareous soils were studied. All the isotherms fit the Freundlich equation. The distribution coefficients (Kf) were low and were only significantly related with the organic matter content of the soils. The main adsorption mechanism at the pH of these soils was found to be by cation bridging between the anionic forms of the two herbicides, and the external surfaces of the organic matter and/or negatively charged clay. The Koc values for mecoprop (R,S-MCPP) and dichlorprop (R,S-DCPP) adsorption predicted a high mobility of these herbicides in the soils. Their corresponding enantiomeric forms, R (biologically active) and S (biologically inactive) were adsorbed at the same ratio on these soils. The organic matter amended soils showed a significant increase on the retention capacity of both molecules.