Effect of phosphate on the adsorption of 2,4-D on lepidocrocite

Abstract

The competitive effect of phosphate on the adsorption of 2,4-D on an iron oxyhydroxide is studied. The adsorption isotherms of the herbicide with no phosphate present are strongly pH-dependent and are concave, suggesting a competition with solvent molecules. The presence of phosphate causes a strong decrease in 2,4-D adsorption and a progressive change in the isotherm from concave to convex. The relationships between the adsorbed amounts of both adsorbates show that in their regions of maximum slope the loss of one site for herbicide adsorption is accompanied by the adsorption of at least 10 phosphate groups, and this number increases as the amount of 2,4-D present decreases. It is shown that the proportion of sites lost for 2,4-D adsorption only depends on the amount of phosphate adsorbed and is independent of the total 2,4-D present in the system, and this conclusion can be explained in terms of the Stern model. The order of addition of both adsorbates has little effect on the final distribution of either adsorbate, although P adsorption seems to be slightly retarded by the presence of 2,4-D previously adsorbed. It is suggested that 2,4-D retention is mainly due to electrostatic forces.