Adsorption of herbicidally active degradate 2-(2,4-dichloro-3-methylphenoxy)propanoic acid on an andosol

The adsorption of 2-(2,4-dichloro-3-methylphenoxy)propanoic acid (DMPA) on the surface horizon of a humus-rich Andosol was examined. To investigate the mechanisms of adsorption, chemically treated Andosols, such as organic matter removed Andosol, organic matter and active metals removed Andosol, and clay minerals of the Andosol, were prepared. Furthermore, humic acid was extracted from the Andosol. The mechanisms of the DMPA adsorption were identified by using those untreated and chemically treated Andosols and the humic acid. The amount of DMPA adsorbed increased with decreasing equilibrium pH value. Active surface hydroxyl groups were identified as the most important soil functional group in DMPA adsorption. The predominant mechanism of DMPA adsorption on the Andosol is a ligand-exchange reaction, in which an active surface hydroxyl on Al and/or Fe is replaced by a carboxylic group of DMPA. A comparative study revealed that the amount of DMPA adsorbed was slightly greater than that of (2,4-dichlorophenoxy)acetic acid (2,4-D), especially at equilibrium pH values below 5. This is because the octanol-water partition coefficient (log Kow) of DMPA in the equilibrium pH range is higher than that of 2,4-D, and SOM participates in the adsorption process through a hydrophobic interaction.