Adsorption of the Herbicide 2,4‐d on Goethite

Summary Adsorption, of the herbicide 2,4‐Dichlorophenoxyacetic acid (2,4‐D) on an iron oxide, goethite, was studied in aqueous suspensions as a function of solution pH, ionic strength of the medium, and initial 2,4‐D concentration. The 2,4‐D anion was reversibly adsorbed on positively charged goethite surfaces, maximum adsorption being observed near the pK a of 2,4‐D (2.73) and at low ionic strength. Within certain levels of adsorption (5–22 mg 2,4‐D adsorbed/g goethite) the complex became hydrophobic and floated to the liquid surface. This flotation effect disappeared on further adsorption. It is suggested that adsorbed 2,4‐D anions are orientated with their hydro‐phobic aromatic ends directed towards the solution, the carboxyl groups being weakly bound to positive sites on the oxide surface. At high levels of adsorption, some of the anions are orientated in the opposite direction by π–π interaction, with the first adsorbed layer and the surface reverts to its hydrophilic nature.